SIMFEROPOL, Ukraine - Russian troops surrounded a military base in Ukraine's Crimea region Sunday in a tense standoff as the security crisis in the region deepened.
Armed men arrived at the base in Perevalne and demanded the Ukrainian soldiers inside lay down their weapons.
The Ukrainian forces remained defiant, but were heavily outnumbered.
No shots were fired in what appeared to be the first direct confrontation between Russian and Ukrainian military forces.
Russian troops haven't fired a shot. They don't need to. With rocket-propelled grenades at the ready, they have taken over Crimea without resistance and with the backing of most of this region's people.
In the regional capital, Simferopol, a pro-Russian self-defense force protected a statue of Lenin.
The base standoff was another sign that Russia was tightening its grip on Crimea – and its influence in other parts of pro-Russian eastern Ukraine.
In Ukraine’s second city, Kharkiv, there were running battles after local militias thwarted pro-Western demonstrators who tried to attack another Lenin statue.
The scenes underscored the problem facing Ukraine's interim leaders, who are powerless to stop the loss of Crimea.
Earlier Sunday, they called up military reservists, put their army on combat alert and appealed to the West for help - but the East of their country is now mutinous.
Russian President Vladimir Putin has so far shown no sign of heeding international demands that he withdraw from Ukraine.
President Barack Obama spoke with Putin by telephone for 90 minutes on Saturday and expressed his "deep concern" about "Russia's clear violation of Ukrainian sovereignty and territorial integrity," the White House said.
NATO's secretary general said Russia had violated the U.N. charter with its military action in Ukraine, and he urged Moscow to "'de-escalate the tensions."
NBC News' Alastair Jamieson contributed to this report from London.