NATO Secretary-General Anders Fogh Rasmussen said on Thursday that an armed group's seizure of regional government headquarters and parliament in Ukraine's Crimea was "dangerous and irresponsible", according to Reuters.

"I am extremely concerned about the most recent developments in Crimea. This morning's action by an armed group is dangerous and irresponsible," Rasmussen told a NATO meeting also attended by Ukraine's acting defense minister.

"I urge Russia not to take any action that could escalate tension or create misunderstanding."