Russia’s military has given Ukrainian forces in Crimea an ultimatum until dawn on Tuesday to surrender or face an assault.
The head of Russia’s Black Sea Fleet Aleksander Vitko set the deadline and also threatened two warships, Ukrainian officials said.
However, Interfax news agency later quoted a fleet spokesman who denied that any ultimatum had been issued.
Moscow has said its troops are needed in Crimea to protect civilians.
The Kremlin says people in Crimea have come under threat from “ultra-nationalists” since pro-Russian President Viktor Yanukovych was ousted last month.
Russia is now said to be in de facto control of the Crimea region.
Ukraine has ordered full mobilization 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.
But 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 PM Arseniy Yatsenyuk said earlier that any attempt to seize Crimea would fail, urging allies to give economic and political support to his government.
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