Ukraine's acting president has warned Russia that troop movements outside its naval base in Crimea would be seen as an "act of aggression". Armed men seized regional government headquarters and parliament and the Russian flag is flying from buildings
US Vice President Joe Biden spoke by phone with Polish Prime Minister Donald Tusk on Thursday to discuss violent protests in Ukraine that have killed 75 people, the White House said.
"They discussed steps the United States, Poland, and the European Union are taking to support an end to the violence and a political solution that is in the best interests of the Ukrainian people," the White House said in a statement.
Ukraine's interim President has expressed concerns about signs of separatism as opposition in Russian-speaking regions grows.
Hundreds of Ukrainians and journalists flocked to the presidential palace, as reports emerged that President Yanukovich had left the city.
Vitali Klitschko was met with boos from anti-government protesters, as he attempted to convince them a peace deal is best course of action.