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.