Speaking to reporters on his flight from Washington to Stuttgart on Tuesday, Carter said NATO ´was considering the deployment a rotational force of about 4,000 troops. These would be in addition to a contingent of 4,200 troops that the US is going to send to Eastern Europe next February.

"That is one of the options that's being discussed," Carter said, adding "We're obviously involved in those discussions."

NATO's Baltic member states Latvia, Lithuania and Estonia have been requesting a stronger alliance presence in the region following Moscow's annexation of Crimea in early 2014.

According to Carter, Russia had 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 presence in Europe, Carter said. The Pentagon chief also said, however, that Washington was willing to "hold the door open if Russian behavior should change," especially in areas where both countries shared benefits, like the Iran nuclear deal.

Scaparotti to take over European NATO reins

The secretary of defense was also set to attend the appointment of Army General Curtis Scaparotti, who will be taking over as the new NATO Supreme Allied Commander of Europe from US Air Force General Philip Breedlove.

In a hearing at the Senate in Washington last month, Scaparotti also said that Moscow was displaying "increasingly aggressive behavior that challenges the international norms, often in violation of international law."

Power play in the Baltics

Carter's trip comes a day after the Pentagon accused Russia of escalating tensions in the Baltic, where a Russian SU-27 conducted a barrel roll over an intelligence-gathering US Air Force RC-135 that was flying over the Baltic Sea.

"I don't think the Russians are trying to provoke an incident. I think they're trying to send a signal," Chief of Naval Operations John M. Richardson told reporters. "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," he added.

Similar incidents happened in April, when a Russian jet flew just 50 feet away from a US aircraft and two Russian jets flew close to USS Donald Cook in the Baltic. At the time, Russian Foreign Minister Sergei Lavrov said that the pilots had decided to take a look at the ship "from a safe distance." US Secretary of State John Kerry responded by saying that the US destroyer could have opened fire.

mg/msh (AP, Reuters)