NATO's top military commander said on Sunday that Russia had a large force on Ukraine's eastern border and said he was worried it could pose a threat to Moldova's mainly Russian-speaking separatist Transdniestria region.

NATO's Supreme Allied Commander Europe, U.S. Air Force General Philip Breedlove, voiced concern about Moscow using a tactic of snap military exercises to prepare its forces for possible rapid incursions into a neighboring state, as it had done in the case of Ukraine's Crimea region.

Russia launched a new military exercise, involving 8,500 artillery men, near Ukraine's border 10 days ago.

"The (Russian) force that is at the Ukrainian border now to the east is very, very sizeable and very, very ready," Breedlove told an event held by the German Marshall Fund think-tank.

The president of ex-Soviet Moldova warned Russia last Tuesday against considering any move to annex Transdniestria, which lies on Ukraine's western border, in the same way that it has taken control of Crimea.

The speaker of Transdniestria's separatist parliament had urged Russia earlier to incorporate his mainly Russian-speaking region.

Transdniestria split away from Moldova in 1990, one year before the dissolution of the Soviet Union, amid fears that Moldova would shortly merge with neighboring Romania, whose language and culture it broadly shares.

Breedlove said NATO was very concerned about the threat to Transdniestria, which he said, in Russia's view, was the "next place where Russian-speaking people may need to be incorporated."

"There is absolutely sufficient [Russian] force postured on the eastern border of Ukraine to run to Transdniestria if the decision was made to do that and that is very worrisome."

Earlier Sunday, Ukraine's acting president said a top air force commander is being held after his base in Crimea was stormed by pro-Russian forces and is calling for his release.

Col. Yuliy Mamchur is the commander of the Belbek Air Force base near Sevastopol, which was taken over Saturday by forces who sent armored personnel carriers smashing through the base's walls and fired shots and stun grenades. One Ukrainian serviceman was reported wounded in the clash. It was unclear if the forces, who didn't bear insignia, were Russian military or local pro-Russia militia.

President Oleksandr Turchynov, in a statement Sunday, said Mamchur was "abducted" by the forces. He didn't specify where Mamchur is believed to be held.

Russia formally annexed the Crimean Peninsula, which is home to Russia's Black Sea Fleet.

