This is Belbek Air Force Base in Crimea, which is still flying the Ukrainian flag. Belbek is not far outside of Sevastopol. According to the BBC, Russian troops have surrounded the base and told the Ukrainians that the deadline to surrender has expired. Ukrainian troops have allegedly been given the order to fire into the air if they are attacked. It is unclear whether the troops have clear orders from Kiev on what to do.
All of this is very worrying, and it's almost bizarre to watch the video of the troops staring at each other, waiting for something to happen.
Update: A Russian armored personnel carrier has now rammed the gate.
Update 2: The live feed has now been disabled after reports of shots being fired on Twitter.
Live feed is here if the embed doesn't work.