Ukrainian troops guard the Belbek air base outside Sevastopol, Ukraine, on Thursday, March 6. Ukrainian officials and Western diplomats accuse Russia of sending thousands of troops into the Crimea region in the past week -- a claim Russia has denied. The crisis in the former Soviet republic has revived concerns of a return to Cold War relationships. Follow the evolving story on CNN's live blog.
Amid the rapidly shifting diplomatic sands, European Union leaders are discussing possible economic and diplomatic sanctions against Russia, which could include asset freezes or visa moves.
U.S. Secretary of State John Kerry spoke at some length with Russian Foreign Minister Sergey Lavrov in Rome on Thursday, on the sidelines of a meeting on Libya.
Martin Schulz, president of the EU Parliament, earlier promised in remarks alongside Yatsenyuk that Europe stood behind the new government in Kiev and a peaceful, democratic future for Ukraine.
"We are behind you and your government, and we support you with all our means," he said.
This includes ensuring that an 11 billion-euro aid package offered Wednesday by the European Union gets to Ukraine as soon as possible to shore up the cash-strapped economy, Schulz said.
Putin, the Russian President, has insisted Russia has the right to use military force in Ukraine if necessary to protect ethnic Russians.
But he has denied claims by Ukrainian officials and Western diplomats that Russia has sent thousands of troops into the region in recent days. Russia says the heavily armed troops, in uniforms without insignia, are local "self-defense" forces.
The deputy speaker of the Crimean parliament, Rustam Temirgaliev, said Thursday at a news conference that the only forces allowed in Crimea are the Russian military -- and that all others will be considered to be occupying forces.
He said he'd advised Ukrainian troops to swear allegiance to the Russian army or leave Crimea under safe passage.
The Organization for Security and Co-operation in Europe said its team of observers, invited into the country by the government in Kiev, had been turned back as they sought to enter Crimea.
Lavrov has criticized moves by the OSCE and NATO, which has suspended some joint undertakings with Russia, saying their actions "are not helping to create an atmosphere of dialogue and constructive cooperation."
In the regional capital, Simferopol, residents have demonstrated this week against the interim government in Kiev, with crowds chanting in favor of Putin.
Tensions remain high around military bases in Crimea, and there are concerns that violence may erupt as tempers fray.
Ukraine's Ministry of Defense said unidentified Russian forces had scuttled an old warship to block seven Ukrainian vessels in a Crimean harbor under cover of darkness Wednesday.
Meanwhile, riot police are in a standoff against pro-Russian demonstrators outside government buildings in Odessa, a port city in southern Ukraine.
And in the eastern city of Donetsk, protesters took over a local government building Wednesday as they called for a referendum on the region's status and greater autonomy, witnesses told CNN.
CNN's Michael Holmes reported from Kiev and Laura Smith-Spark wrote and reported in London. CNN's Anna Coren in Simferopol, Matthew Chance in Odessa and Tim Schwarz in Kiev contributed to this report. CNN's Elise Labott, Michelle Kosinski, Susan Garraty, Susannah Palk and Yon Pomrenze also contributed.
The BBC's Andrew Harding reports on the latest developments in the Oscar Pistorius trial
A neighbour of Oscar Pistorius has told his murder trial he found the South African Paralympic star praying over the body of his girlfriend Reeva Steenkamp as she lay dying.
In the trial's first account of the moments after the shooting, Johan Stipp said he tried to revive Ms Steenkamp.
During Dr Stipp's testimony, Mr Pistorius cried and seemed to retch.
The double amputee denies intentionally killing her, saying he mistook the 29-year-old model for an intruder.
If found guilty, the 27-year-old could face life imprisonment.
Dr Stipp said he could see Ms Steenkamp's brain tissue in her hair.
At this point, a police officer passed a plastic bag to Mr Pistorius.
Recalling the night of 14 February 2013, Dr Stipp said he had heard gunshots.
He said he had found Mr Pistorius kneeling by her body.
"I shot her," the Paralympic star told him. "I thought she was a burglar."
"Oscar was crying all the time,'' Dr Stipp said. "He was praying to God [saying] `Please let her live.'"
Pistorius said he would dedicate "his life and her life to God" if she lived, according to the testimony.
Dr Stipp said Mr Pistorius had been in an emotional state. Fearing he might hurt himself, he had asked where the gun was.
The trial has been adjourned until Friday.
Earlier, defence lawyer Barry Roux said that two neighbours, Michell Burger and Charl Johnson, had discussed their testimony with each other and amended the rough notes they had made a few days after the shooting.
"Your interpretation [of events] is designed to incriminate the accused and it's unfortunate," Mr Roux said.
Mr Johnson denied this. He insisted that he had heard a woman screaming "help", followed by a man doing the same.
On Wednesday, a witness said the athlete had been responsible for a weapon being fired at a restaurant last year.
Boxer Kevin Lerena told the court Mr Pistorius had asked the owner of the gun to take the blame after the incident in January 2013.
Mr Pistorius also faces charges of illegally possessing ammunition, which he denies.
Ms Steenkamp, a model and reality TV star, was shot dead in the early hours of Valentine's Day last year at Mr Pistorius' home in Pretoria.
Oscar Pistorius cried in court as he heard testimony from a neighbour
The arrest of a national sporting hero, who won gold at the London 2012 Paralympic Games and also competed at the Olympics, astounded South Africa.
Much of the case will depend on ballistic evidence from the scene of the shooting, correspondents say.
The state is seeking to convince the court that Oscar Pistorius and Reeva Steenkamp had an argument before the athlete fired the shots that killed his girlfriend.
South Africa was stunned when the athlete shot dead his girlfriend, Reeva Steenkamp