A uniformed man, believed to be a Russian serviceman, stands guard near a Ukrainian military base outside the city of Sevastopol, March 7, 2014. President Vladimir Putin rebuffed a warning from U.S. President Barack Obama over Moscow's military intervention in Crimea, saying on Friday that Russia could not ignore calls for help from Russian speakers in Ukraine. REUTERS/David Mdzinarishvili

Update 4 p.m. EST: The standoff has reportedly ended, but not before journalists report colleagues being assaulted by "self-defense" forces and supporters outside the bases gates.

Original story: Armed men wearing Russian uniforms without insignia have stormed and taken control of part of a Ukrainian air force base in Sevastopol, Crimea, the Kiev government and witnesses say.

No shots have been fired and reports say the two sides are now negotiating.

The base is an anti-aircraft command center. The armed men rammed the gates with a truck and demanded the Ukrainian forces give up their arms and surrender.

The Ukrainian Ministry of Defense in Kiev says there are 100 Ukrainians in the base who are reportedly holed up in a bunker.

Russian officials have repeatedly claimed the apparent Russian troops in the Crimea are actually a “Crimean self-defense force,” but soldiers have unofficially confirmed they are Russian. Cossacks are other “military irregulars” are reportedly preventing reporters from getting video or photos of the base.

Armed men have surrounded Ukrainian bases for a week now but have not broken into any bases until now.

A Time magazine reporter tweeted around 2:30 p.m. EST that 15 trucks carrying Russian troops landed via a Black Sea Fleet ship, although it is not confirmed that these are the same troops besieging the Ukrainian air force base.