BERLIN (AP) — NATO says it's sent two surveillance planes to fly over Poland and Romania to monitor the crisis unfolding in neighboring Ukraine.

The military alliance says two AWAC, or Airborne Warning And Control System, reconnaissance planes took off from its bases in Germany and Britain this morning.

A NATO spokesman says the flights will not leave the air space of its member nations — meaning they won't cross into either Ukrainian or Russian airspace.

NATO's 28 member states decided Monday to intensify the assessment of the possible threat the Ukrainian crisis poses to the alliance.

Ukrainian Prime Minister Arseniy Yatsenyuk (ahr-SEHN'-ee yaht-sehn-YOOK') will meet today in Washington with President Barack Obama.

Yatsenyuk has called on the West to defend Ukraine against Russia, which he says "is armed to the teeth" and "has nuclear weapons."

He has asked Russia, the U.S. and Britain to abide by a treaty signed in 1994, in which they pledged to guarantee Ukraine's security in exchange for surrendering its share of Soviet nuclear arsenals to Russia.