Hundreds of unidentified gunmen have arrived outside Ukraine's infantry base in Privolnoye in its Crimea region, Associated Press journalists said.
The convoy includes at least 13 troop vehicles each containing 30 soldiers and four armored vehicles with mounted machine guns. The vehicles -- which have Russian license plates -- have surrounded the base and are blocking Ukrainian soldiers from entering or leaving it.
Ukrainian soldiers, with clips in their weapons, have positioned a tank at the gate.
Ukraine's acting Prime Minister said Sunday that his country was "on the brink of disaster" and called for world leaders to take "real steps" to defuse the ongoing crisis in the Crimean Peninsula even as Russian troops appeared to be advancing toward the provincial capital.
Arseniy Yatsenyuk spoke after a closed session of his new parliament in Kiev, calling on Russian President Vladimir Putin to "pull back his military."
Earlier Sunday, Andriy Paruby, Secretary of Ukraine's Security Council, which is made up of Ukraine's top security and defense chiefs, announced that all military reservists were being called up to active duty and added that it was vital to ensure that the Ukrainian armed forces were combat-ready as soon as possible. Paruby also said that Ukraine's Foreign Ministry had been ordered to seek assistance from the U.S. and Great Britain in guaranteeing the country's safety.
Associated Press journalists witnessed a convoy of 12 military trucks carrying troops, a Tiger vehicle armed with a machine gun, and two ambulances traveling along the road from Sevastopol, the home of Russia's Black Sea Fleet, to the regional capital, Simferopol. There have been no reports of Russian troops meeting any resistance in either landing or moving through Crimea.
NATO's North Atlantic Council, the alliance's political decision-making body, and the NATO-Ukraine Commission were to meet on Sunday. NATO Secretary-General Anders Fogh Rasmussen said the allies will "coordinate closely" on the situation in Ukraine, which he termed "grave."
Ukraine is not a NATO member, meaning the U.S. and Europe are not obligated to come to its defense. But Ukraine has taken part in some alliance military exercises and contributed troops to its response force.
On Saturday, Putin sought and received unanimous consent from the Russain parliament to mobilize the country's military in Ukraine. But while sometimes-violent pro-Russian protests broke out Saturday in a number of Russian-speaking regions of eastern Ukraine, Moscow's immediate focus appeared to be only Crimea.
Ukraine's interim President, Oleksandr Turchynov said in an address to the Ukrainian nation Saturday that he had also ordered increased security at nuclear power plants, airports and other strategic infrastructure, because of the threat of "potential aggression." Appearing beside him, Yatsenyuk said: "I am convinced that Russia will not launch an intervention as this would mean war." Yatsenyuk added that he had urged withdrawal of Russian troops in a phone conversation with his Moscow opposite, Dmitry Medvedev.
Lawmakers in Moscow also asked that the country's ambassador in Washington be recalled after earlier statements by President Obama.
After Russia's parliament approved Putin's motion, U.S. officials held a high-level meeting at the White House to review Russia's military moves in Ukraine. The White House said Obama spoke with Putin by telephone for 90 minutes and expressed his "deep concern" about "Russia's clear violation of Ukrainian sovereignty and territorial integrity."
Putin said that the mobilization is needed to protect ethnic Russians and the personnel of a Russian military base in Ukraine's strategic region of Crimea. But the request came a day after Obama warned Moscow that "there will be costs" if it intervenes militarily in Ukraine.
The White House said Obama told Putin that the United States is calling on Russia "to de-escalate tensions by withdrawing its forces back to bases in Crimea and to refrain from any interference elsewhere in Ukraine."
A statement from the Kremlin said Putin emphasized to Obama the existence of "real threats" to the life and health of Russian citizens and compatriots who are in Ukrainian territory. The statement indicated that Russia might send its troops not only to the Crimea but also to predominantly ethnic Russian regions of eastern Ukraine.
"Vladimir Putin emphasized that, in the case of a further spread in violence in eastern regions (of Ukraine) and Crimea, Russia maintains the right to protect its interests and the Russian-speaking population that lives there," the Kremlin statement said.
Obama told Putin that he would support sending international monitors to Ukraine to help protect ethnic Russians. He said the U.S. will suspend its participation in preparatory meetings for June's G-8 summit in Sochi, Russia, the site of the recently concluded Winter Olympics, warning that Russia's "continued violation of international law will lead to greater political and economic isolation."
French Foreign Minister Laurent Fabius agreed, saying on French radio Europe that planning for the summit should be put on hold. France "condemns the Russian military escalation" in Ukraine, and Moscow must "realize that decisions have costs," he said Sunday.
WATCH: Is it too late to warn Russia to stay out of Ukraine?
The U.N. Security Council met in an open, televised session for about a half hour on Saturday afternoon after closed-door consultations, despite initial objections from Russia to an open session. The council heard speeches from a U.N. deputy secretary-general and several ambassadors, but did not take any action.
Ukraine's Ambassador to the U.N. Yuriy Sergeyev asked the Security Council "to do everything possible now" to stop what he called Russian "aggression." Russia's U.N. ambassador, Vitaly Churkin, said the government in Kiev needs to get away from "radicals" and warned, "such actions they're taking could lead to very difficult developments, which the Russian Federation is trying to avoid." He said Russia was intervening at the request of pro-Russian authorities in the autonomous Crimea region that is part of Ukraine.
Calling the situation in Ukraine "as dangerous as it is destabilizing," U.S. Ambassador to the U.N. Samantha Power said, "It is time for the Russian military intervention in Ukraine to end." She warned that "Russia's provocative actions could easily push the situation beyond the breaking point." She asked that Russia directly engage the Ukraine government and called for international monitors to be sent to Ukraine to observe the situation.
"Russia and the West find themselves on the brink of a confrontation far worse than in 2008 over Georgia," Dmitri Trenin, the director of Carnegie Moscow Center, said in a commentary posted on its website. In Georgia, Russian troops quickly routed the Georgian military after they tried to regain control over the separatist province of South Ossetia that has close ties with Moscow.
WATCH: Krauthammer 'shocked' by Obama Ukraine statement
The latest moves followed days of scripted, bloodless turmoil on the peninsula, the scene of centuries of wars and seen by Moscow as a crown jewel of the Russian and Soviet empires. What began Thursday with the early-morning takeover of the regional parliament building by mysterious troops continued Saturday afternoon as dozens of those soldiers — almost certainly Russian — moved into the streets around the parliamentary complex and seized control of regional airports, amid street protests by pro-Russian Crimeans calling for Moscow's protection from the new government in Kiev.
That government came to power last week in the wake of months of pro-democracy protests against the now-fugitive president, Viktor Yanukovych, and his decision to turn Ukraine toward Russia, its longtime patron, instead of the European Union. Despite the calls for Moscow's help, there has been no sign of ethnic Russians facing attacks in Crimea or elsewhere in Ukraine.
Obama on Friday called on Russia to respect the independence and territory of Ukraine and not try to take advantage of its neighbor's political upheaval.
He said such action by Russia would represent a "profound interference" in matters he said should be decided by the Ukrainian people. He has not said, however, how the U.S. could pressure Moscow to step back from its intervention.
The Russian parliament urged that Moscow recall its ambassador in Washington in response to Obama's speech.
On Friday, Ukraine accused Russia of a "military invasion and occupation" in the Crimea, where Russia's Black Sea fleet is based. Ukrainian Prime Minister Arseny Yatsenyuk called on Moscow "to recall their forces, and to return them to their stations," according to the Interfax news agency. "Russian partners, stop provoking civil and military resistance in Ukraine."
U.S. officials told Fox News Friday that they saw "evidence of air and maritime movement into and out of Crimea by Russian forces," although the Pentagon declined to officially "characterize" the movement.
Ukraine's population of 46 million is divided in loyalties between Russia and Europe, with much of western Ukraine advocating closer ties with the European Union while eastern and southern regions look to Russia for support. Crimea, a semi-autonomous region that Russia gave to Ukraine in the 1950s, is mainly Russian-speaking.
In his address to parliament, Putin said the "extraordinary situation in Ukraine" was putting at risk the lives of Russian citizens and military personnel stationed at the Crimean naval base that Moscow has maintained since the Soviet collapse.
Despite Putin's sharp move, there were possible signs Saturday that the Russian leader could soften his approach. Former Ukrainian Prime Minister Yulia Tymoshenko, who was freed a week ago after more than 2 ½ years in prison, was reported to be heading to Moscow for a meeting with Putin on Monday, though her spokeswoman denied that. Putin has had good ties with Tymoshenko in the past, and he may look to her for a possible compromise.
In a statement posted on her party's web site, Tymoshenko urged the U.N. Security Council to meet in Kiev and asked the EU leaders to convene a meeting in Crimea. She urged the West to help protect Ukraine's territorial integrity, asked Ukrainians to remain calm and voiced hope that diplomacy will succeed.
Putin's parliamentary motion loosely refers to the "territory of Ukraine" rather than specifically to Crimea, raising the possibility that Moscow could use military force in other Russian-speaking areas in eastern and southern Ukraine, where many detest the new authorities in Kiev.
But in a note of restraint, Deputy Foreign Minister Grigory Karasin said the motion doesn't mean the president would immediately send additional troops to Ukraine. "There is no talk about it yet," he said.
Pro-Russian protests were reported Saturday in the eastern cities of Kharkiv, Donetsk and Luhansk and the southern port of Odessa. In Kharkiv, 97 people were injured in clashes between pro-Russia demonstrators who flushed supporters of the new Ukrainian government out of the regional government building and hoisted the Russian flag on top of it, according to the Interfax news agency.
Trenin, of Moscow's Carnegie office, said that Putin could be seeking to "include Crimea within the Russian Federation and eastern and southern regions of Ukraine forming a separate entity integrated with Russia economically and aligned with it politically."
"It is not clear at this point whether Kiev will be left to build a rump Ukraine with the western regions or whether it will be swayed to join the eastern regions," he wrote.
In Crimea, the new pro-Russian prime minister — who came to power after the gunmen swept into parliament on Thursday — claimed control of the military and police and asked Putin for help in keeping peace. There was no visible presence of Ukrainian troops Saturday.
The deputy premier in the Crimean government told Russian news agency RIA Novsti that Ukrainian troops were disarmed and others joined the Crimean people to help patrol the territory. The report couldn't immediately be confirmed.
Crimean Tatars, the historic hosts of the land who make up 12 percent of the island's population and stand strongly for Crimea remaining part of Ukraine, didn't put up any visible resistance Saturday.
"The last two or three days have turned around the life of all the people in Crimea," said Refat Chubarov, a Crimean Tatar leader. "They've taken over military bases and civil institutions. That's why Crimean society is filled with fear. People are afraid of everyone and everything."
Crimea only became part of Ukraine in 1954 when Soviet leader Nikita Khrushchev transferred jurisdiction from Russia, a move that was a mere formality when both Ukraine and Russia were part of the Soviet Union. The Soviet breakup in 1991 meant Crimea landed in an independent Ukraine.
Swedish Foreign Minister Carl Bildt summed the situation up simply: "What's happening in Crimea is a Russian takeover. There is no doubt about that," he told Swedish Radio. "Russian military forces are involved and there has been a local takeover of power."
Russia put pressure on Ukraine from another direction when a spokesman for state gas company Gazprom said that Ukraine owed $1.59 billion in overdue bills for imported gas. Sergei Kuprianov said in a statement carried by Russian news wires that the gas arrears would endanger a recent discount granted by Russia.
The Russian payment demand and loss of the discount would accelerate Ukraine's financial crisis. The country is almost broke and seeking emergency credit from the International Monetary Fund.
The tensions barely touched everyday life in Simferopol, the regional capital of Crimea, or anywhere on the peninsula. Children played on swings a few blocks from the parliament building, and most of the city's stores were open. Couples walked hand-in-hand through parks. Crimea's airports — civilian and military — were closed to air traffic, but trains and cars were moving to and from the Ukrainian mainland. The civilian airport in Simferopol was reopened late Saturday night.
"Things are normal," said Olga Saldovskaia, who was walking through town with her son and grandson. While she doesn't like having gunmen in the streets, like many people in this overwhelmingly ethnic Russian city, she also found their presence reassuring.
"If anyone tries to hurt the people here, they will protect us," said Saldovskaia. She said she sympathized with the pro-democracy protesters in Kiev, but also worries that turmoil in the capital could lead to violence against ethnic Russians. She added, though, that she definitely doesn't want Crimea to become part of Russia.
"Russia is not just all flowers and candy," she said.
Moscow has remained silent on claims that Russian troops are already in control of much of the peninsula, saying any troop movements are within agreed-upon rules governing the semi-autonomous Ukrainian region.
Meanwhile, flights remained halted at Simferopol's airport. Dozens of armed men in military uniforms without markings patrolled the area. They didn't stop or search people leaving or entering the airport, and refused to talk to journalists.
Associated Press journalists crossing into Crimea from mainland Ukraine were briefly stopped at a checkpoint manned by troops in unmarked camouflage uniforms as well as officers in uniforms of the Berkut, the feared riot police that cracked down on anti-Yanukovych protesters before he fled the capital a week ago.
The Associated Press contributed to this report.
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Armed men patrol outside the Simferopol International Airport in Ukraine's Crimea region on Friday, February 28. The gunmen, whom Ukrainian Interior Minister Arsen Avakov called part of an "armed invasion" by Russian forces, appeared around the airport without identifying themselves. Crimea is an autonomous republic of Ukraine with an ethnic Russian majority. It's the last large bastion of opposition to Ukraine's new political leadership after President Viktor Yanukovych's ouster. 