People take photos with U.S. soldiers in Bialystok, Poland last year. (Alik Keplicz/AP)

STUTTGART, Germany — NATO is considering placing thousands of additional troops in Poland and the Baltic states, Defense Secretary Ashton B. Carter said Monday, adding that any new forces in the region would be rotational and part of an effort to deter future Russian aggression.

The Pentagon chief would not specify what countries would contribute troops but said the possible deployment is one of several options being weighed by the alliance. Any final decision to bolster troop levels in Europe will probably be made at a NATO summit in Warsaw this summer, he said.

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Some aspects of the possible deployment were first reported in the Wall Street Journal on Friday. According to the report, the contingent could comprise four battalions of roughly 1,000 troops apiece. Western officials told the Journal that two of the battalions would come from the United States and the other two from Germany and Britain. But the officials said the numbers and contributions have not been finalized.

The new NATO presence would be in addition to the recently formed Very High Readiness Joint Task Force. The task force, created last year, is composed of 5,000 troops from various NATO countries and is designed to act as a quick response unit for the alliance.

“There are a number of bilateral initiatives that the U.S. has put forward to increase security and defense in Europe, but there are also a number of efforts going on under a NATO context to increase deterrence and defense,” said a senior U.S. defense official who spoke on the condition of anonymity to talk about military planning.

A significant addition of troops was discussed and agreed upon in principle during a NATO meeting in February, but many of the details had yet to be ironed out.

“We will have as much presence in the East as needed,” NATO Secretary General Jens Stoltenberg said at the time.

Carter’s indication that NATO might be willing to commit more resources to Eastern Europe comes as he visits Germany to oversee the changeover of European Command’s top general. U.S. Army Gen. Curtis Scaparrotti is set to replace Air Force Gen. Philip M. Breedlove in a Tuesday ceremony here. Carter is expected to make a series of remarks aimed at reassuring U.S. allies that the Pentagon will help Europe confront challenges including the waves of migrants that have poured into the European Union and some of Russia’s actions.

In recent weeks, Russian aircraft have intercepted at least one U.S. RC-135 reconnaissance aircraft and buzzed a U.S. destroyer operating in the Baltic Sea. In April, two unarmed Russian Su-24 multi-role fighters performed multiple close passes over the USS Donald Cook. At one point the Russian aircraft came within 30 feet of the ship’s deck. U.S. officials have called the flybys unprofessional, while the Russian Defense Ministry has played down the incidents.

In February, the Pentagon announced it was quadrupling the budget for its European Reassurance Initiative to $3.4 billion for fiscal 2017. The new funds will support the continuous presence of three U.S. Army brigades of roughly 2,500 soldiers each in Eastern Europe by the end of 2017. Earlier parts of the Reassurance Initiative involved the positioning of U.S. equipment in Europe for deploying units to use for training. Under the current program, the equipment will be modernized and can be used for combat, a move that Pentagon officials say pushes the scope of the initiative from “reassurance to deterrence.”

Currently there are roughly U.S. 60,000 troops stationed in Europe, with an additional 4,000 rotational troops.