Russian forces in Crimea demanded Monday evening that the crews of two Ukrainian warships surrender within the hour or face an assault on their ships and crew, according to a Ukrainian official.

Four Russian navy ships were in position around the Ukrainian ships in Sevastopol’s harbor as they awaited a response to the surrender ultimatum, the Associated Press reports. Ukrainian defense sources also told the BBC Russia had set a 03:00 GMT deadline for Ukrainians soldiers in Crimea to surrender or be attacked “across Crimea,” but a Ukrainian spokesman could not confirm that order.

Russian troops have encircled Ukrainian military bases throughout Crimea and are in a tense standoff that has thus far not resulted in bloodshed. But any Russian ultimatum could lead the nations to violence after Ukrainian authorities announced it would “never give up” Crimea.

The standoff began over the weekend after Russian President Vladimir Putin was given approval by his parliament to send troops to Ukraine, shortly after unidentified pro-Russia militants wearing fatigues first appeared around key locations in the Crimean region last week. Russian forces now control all Ukrainian border posts, as well as all military facilities.

Putin has claimed Russia needs to protect large ethnic Russian populations on the peninsula. But there have been no reports of organized hostility toward Russian speakers in Ukraine during the four-month upheaval that shook pro-Russian President Viktor Yanukovych from power.

Russia’s invasion of Ukraine has caused a global uproar as Western leaders scramble to respond to Putin’s aggression. President Obama and Congress are currently considering economic sanctions and other measures that would isolate Russia but have yet to set a clear path.

[AP]

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