Defense Secretary Ash Carter leaves after a news conference Thomson Reuters 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 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 Tuesday at a ceremony installing a new commander of US European Command.

Army Gen. Curtis Scaparrotti is to replace Air Force Gen. 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.

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

Russia has accused the US and NATO of returning to a Cold War mindset of mutual suspicion and military competition, even as it continues to buzz US ships and planes in the Baltics.

Speaking more broadly of US and NATO relations with Russia, Carter said Moscow has chosen to move away from integration with the West. "Therefore, we have no alternative but to do what we're doing, which is stand strong," by improving the US military posture in Europe and collaborating closely with NATO allies, he said.

NATO military training at Ādaži Military Base. NATO

At the same time, Carter said, the US is willing to "hold the door open if Russian behavior should change" and to work with Russia in areas where the two countries still have mutual interests, such with the Iran nuclear deal.

In his remarks en route to Stuttgart, Carter also called the buzzing of US Navy ships and aircraft in the Baltics "unprofessional," adding that it seems to be happening more frequently.

"This kind of unprofessional behavior by its nature creates a dangerous circumstance," he said.

At the Pentagon on Monday, the Navy's top officer said the Russian actions in the Baltics are escalating tension between the two nations.

"My hope is that we can stop this sort of activity," Adm. John M. Richardson, the chief of naval operations, told reporters.

Russian Sukhoi SU-24 attack aircraft makes low pass close to the U.S. guided missile destroyer USS Donald Cook in the Baltic Sea Thomson Reuters

"I don't think the Russians are trying to provoke an incident. I think they're trying to send a signal," he said. "I think it's pretty clear that they are wanting to let us know that they see that we are up there in the Baltic."

The Defense Department said a Russian SU-27 conducted a barrel roll Friday over a US Air Force RC-135 that was flying a reconnaissance mission above the Baltic Sea. The RC-135 is an intelligence-gathering aircraft.

In mid-April, a Russian jet flew about 50 feet from the wing tip of a US aircraft. Also in April, two Russian jets flew close to the USS Donald Cook in the Baltic Sea.

Richardson said the actions increase the chance of a "tactical miscalculation," but that if an incident were to occur, the US would tamp down any rise in tensions between the two countries.

An U.S. Navy picture shows what appears to be a Russian Sukhoi SU-24 attack aircraft flying over the U.S. guided missile destroyer USS Donald Cook in the Baltic Sea Thomson Reuters

"We look for sort of a normalization there," he said.

Russian Foreign Minister Sergey Lavrov last week said the pilots decided to take a look at the US Navy destroyer "from a safe distance." The planes were less than 100 feet away from the deck of the ship, traveling at hundreds of miles per hour.

Secretary of State John Kerry condemned the Russian actions and said the Navy ship could have opened fire.