Russia's military has given Ukrainian forces in Crimea until dawn on Tuesday to surrender or face an assault, Ukrainian defence sources have said.The head of Russia's Black Sea Fleet Aleksander Vitko set the deadline and threatened an attack "across Crimea".
He also reportedly told two warships to surrender or be attacked at 17:00 GMT on Monday.
Moscow says it is protecting civilians from "ultra-nationalist threats", but its actions have been widely condemned.
Russia is now said to be in de facto control of the Crimea region.
Ukraine has ordered full mobilisation to counter the intervention.
No shots have yet been fired in the region, which has a majority of Russian speakers and a largely pro-Russian local government.
However, the captain of one of the threatened warships told Ukrainian TV his men were prepared to fight and would not surrender.
The trouble began last month when pro-Russian President Viktor Yanukovych was ousted following months of street protests.
Russia claims its military is protecting human rights in Crimea, but Kiev, the US and western Europe have condemned the actions.
Ukraine's Prime Minister Arseniy Yatsenyuk said earlier that any attempt to seize Crimea would fail, urging allies to give economic and political support to his government.