Ships began unloading U.S. tanks, self-propelled howitzers and hundreds of other fighting vehicles Friday in the northern German port of Bremerhaven, to be moved into Eastern Europe to bolster NATO's deterrence against possible Russian aggression.

Some 3,500 troops from the 4th Infantry Division in Fort Carson, Colorado, will join up with the equipment, which includes 87 tanks and 144 Bradley fighting vehicles, over the next two weeks.

The deployment marks the start of a new phase of Operation Atlantic Resolve, which foresees the continuous presence of an American armored brigade combat team in Europe on a nine-month rotational basis. The mission is meant to help allay concerns from Estonia, Latvia, Lithuania , Poland and other NATO allies over an increasingly unpredictable and bellicose Russia.

The new forces will gather first in Poland, then fan out across seven countries from Estonia to Bulgaria. A headquarters unit will be stationed in Germany.

When he announced the move last year, U.S. Defense Secretary Ash Carter said the forces would take part in regular military exercises across the region with NATO allies. At that time, U.S. Army Europe Commander Lt. Gen. Ben Hodges called the deployment the "embodiment of the United States' commitment to deterring aggression and defending our European Allies and partners."

The U.S. also plans to move in a combat aviation brigade with about 10 Chinook and 50 Black Hawk helicopters and 1,800 personnel from Fort Drum, New York, and a battalion with 24 Apache attack helicopters and 400 personnel from Fort Bliss , Texas. They'll be headquartered in Germany with some aircraft positioned in Latvia, Romania and Poland.

Other NATO members are also increasing their presence, with Britain sending fighter jets to the Black Sea area, while a battalion of troops, tanks and light armor will deploy in Estonia in the spring, backed by French and Danish troops. Germany also plans to send troops and tanks to Lithuania.

Albania, Belgium, Canada, Croatia, France, Italy, Luxembourg, the Netherlands, Romania and Slovenia are also playing roles in what NATO has dubbed its Enhanced Forward Presence. The U.S. plans to relocate a Stryker unit from Germany to Poland as part of that group.

NATO has already started positioning equipment and ammunition in Eastern Europe to reduce the time it would take additional units to deploy if needed.