Ukraine's acting president has warned the Russian navy in Crimea not to leave its post, after gunmen seized government buildings in the regional capital.
Oleksandr Turchnyov said Thursday that any movement of Russian troops will be considered "military aggression." Russia's Black Sea fleet is based in the Crimean city of Sevastpol.
Earlier Thursday, Ukraine's acting interior minister Arsen Avakov said police are on high alert after the local government and legislature buildings were seized in the capital, Simferopol, by armed men in the pro-Russia region.
Reports say the the Russian flag was raised over the buildings, and Avakov said the occupied buildings have been sealed off by police.
The area has been the scene of confrontations between supporters of Ukraine's new government and pro-Russia activists. The region is mainly made up of Russian speakers who support Moscow but is also home to minority Muslim Tatars who are generally anti-Russia.
On Wednesday, Ukrainian interim leaders named popular opposition figure Arseniy Yatsenyuk as the country's new prime minister. Parliament is expected to consider his name later Thursday.
Yatsenyuk is a pro-Western former foreign minister and economy minister. One of his first major jobs would be preventing the Ukrainian economy from collapse.
The White House says it strongly supports Ukrainian leaders as they work to form a multiparty government to represent all Ukrainians. It calls a broad-based government committed to reconciliation the necessary foundation for international aid.
Secretary of State John Kerry says the U.S. is considering $1 billion in loan guarantees for Ukraine.
Kerry also warned Russia Wednesday that it would be a "grave mistake" to intervene militarily in Ukraine.
Russian President Vladimir Putin has ordered an urgent military drill in areas that include the region near the Ukrainian border.
Russian Defense Minister Sergei Shoigu says the drill will check "readiness to deal with crisis situations that threaten the nation's military security."
The Kremlin did not superficially mention Ukraine. It has said it would not interfere in Ukraine's internal affairs. Moscow is a strong supporter of ousted Ukrainian President Viktor Yanukovych.
Ukraine is split between the pro-European west and the pro-Russian east and south.
Anti-government demonstrations erupted in Ukraine when Mr. Yanukovych backed out of a European Union trade deal in favor of economic aid from Russia. The violence escalated last week, leaving nearly 100 people dead.
Ukraine's interim leaders have dissolved an elite security force accused of carrying out some of those deadly attacks on protesters.