Ukrainian military base in Crimea under siege

(CNN) – The standoff is beginning to boil over in Ukraine's Crimea peninsula after days of tension.

The Ukrainian military says that a group of Cossacks smashed the gates of a base that controls all the airspace in southern Ukraine, using a Russian military truck. Some Ukrainian officers are said to be barricaded inside the control room. Others are reportedly trying to negotiate with the force outside, which still claims to be a Crimean self-defense group, not Russian.

CNN's Matt Chance reports from Ukraine.