Ukraine says Russian forces controlling Crimea have demanded the surrender of two warships, a claim Russia strenuously denied.
As the crisis deepened top U.S. legislators called for sanctions on Russia, including measures targeting banks and officials. President Obama said the U.S. was examining economic and diplomatic steps to isolate Moscow, and he called on Congress to expedite assistance for Ukraine.
Ukrainian Defense Ministry spokesman Maksim Prauta said four Russian navy ships were blocking Ukraine's anti-submarine warship Ternopil and the command ship Slavutych in Sevastopol's harbor.
He said the Russians ordered the crew to surrender within the hour or face Russians storming and seizing the ships and crew.
Interfax also quoted an unnamed source in the Ukrainian Defense Ministry earlier on Monday as saying a deadline to surrender at 0300 GMT had been set by the Black Sea Fleet's commander.
The same news agency later quoted an unnamed representative at the fleet's headquarters as saying no assault was planned, adding: "This is complete nonsense.''
The State Department said the reports, if true, would be "dangerous escalation" for which it would hold Russia responsible.
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