U.S. Defense Secretary Ash Carter says the NATO alliance is considering establishing a rotational ground force in the Baltic states and possibly Poland as a deterrent to Russian aggression.

Carter made the comment to reporters flying with him Monday to Stuttgart, Germany, home of the U.S. European Command.

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 U.S. armored brigade of about 4,200 troops that the U.S. has already committed to deploying to Eastern Europe next February.

Carter said the idea of a separate NATO rotational ground force is likely to be further discussed at a NATO meeting in June.