According to Reuters, Russia's Black Sea Fleet has told Ukrainian forces in Crimea to surrender by 5 a.m. (0300 GMT) on Tuesday or face a military assault, Interfax news agency quoted a source in the Ukrainian Defence Ministry as saying.
The ultimatum, Interfax said, was issued by Alexander Vitko, the fleet's commander.
The ministry did not immediately confirm the report and there was no immediate comment by the Black Sea Fleet, which has a base in Crimea, where Russian forces are in control.
"If they do not surrender before 5 a.m. tomorrow, a real assault will be started against units and divisions of the armed forces across Crimea," the agency quoted the ministry source as saying.
Russia has staged war games with 150,000 troops along the land border, but so far they have not crossed. Kiev says Moscow is orchestrating the protests to justify a wider invasion.
Ukraine's security council has ordered all armed forces to be put on highest alert. However, Kiev's small, under-equipped military is seen as no match for Russia's superpower might.
While the EU and NATO stepped up verbal pressure on Moscow, a German spokesman said Merkel believed it was not too late to resolve the Ukrainian crisis by political means despite differences of opinion between Putin and the West.
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Stephen "TheeBlackSocialite" Hale