The NATO alliance is weighing rotating four battalions of troops through Eastern member states, U.S. Defense Secretary Ash Carter said on Monday, in the latest proposal by allies to guard against aggressive behavior by Russia.

The Baltic states — Estonia, Latvia and Lithuania — which joined NATO in 2004, have requested greater presence of the alliance, fearing a threat from Russia after it annexed the Crimea peninsula from Ukraine in 2014.

Carter acknowledged NATO deliberations included the deployment of the four battalions to the Baltic states and Poland. The Wall Street Journal said this would likely total about 4,000 troops split between the United States and its allies.

"That's one of the options that's being discussed," Carter told reporters traveling with him at the start of a three-day trip to Germany, declining to enter into details about the deliberations by the North Atlantic Treaty Organization.

"We're obviously involved in those discussions. I just don’t want to get out in front of where that goes."

U.S. officials say the goal in Europe is to move increasingly from efforts to reassure allies to broader activity to deter any aggressive moves by Russia.

The United States has already budgeted to sharply boost military training and exercises and last month announced it would deploy continuous rotations of U.S.-based armored brigade combat teams to Europe.

Carter's trip to Germany will include meetings with Army Gen. Curtis Scaparrotti as he takes over as the next NATO Supreme Allied Commander Europe, succeeding U.S. Air Force Gen. Philip Breedlove.

Scaparrotti told a Senate hearing last month that a resurgent Russia was displaying “increasingly aggressive behavior that challenges the international norms, often in violation of international law.”