The NATO alliance is considering establishing a rotational ground force in the Baltic states and possibly Poland, reflecting deepening worry about Russian military assertiveness, US Defense Secretary Ash Carter said on Monday.

"That is one of the ideas that's under discussion," Carter told reporters flying with him from Washington to Stuttgart, Germany, where he is to preside on Tuesday at a ceremony installing a new commander of US European Command. Army General Curtis Scaparrotti is to replace Air Force General Philip Breedlove, who has frequently and publicly cautioned that Russia poses a potential threat to European stability.

Carter said the allies are considering a rotational ground force of four battalions, which would mean about 4,000 troops. That would be in addition to, and separate from, a recently announced unilateral US decision to send a US armored brigade of about 4,200 troops to Eastern Europe next February.