The US military is preparing for the largest deployment of forces to Europe in 25 years as thousands of troops are attending massive war games amid rising tensions with Moscow.

About 20,000 American troops based in the US will be deployed next year to Europe where they will join some 9,000 other US soldiers already stationed there, said General Christopher Cavoli, the commander of US ground forces in Europe.

The Defender Europe-20 exercise will link several exercises in the region, such as Allied Spirit, Swift Response and a Joint Warfighting Assessment. Nearly 37,000 troops will then take part in the exercise, which will take place across 10 European countries from May to June, Cavoli told reporters at the US Defense Department.

The US-based forces will begin arriving in February, moving 13,000 pieces of equipment, including tanks, artillery and transport vehicles, across 2,500 miles (4,000 kilometers), he said.

While Cavoli did not single out Russia as the focus, he said its reunification with Crimea in 2014 changed everything.

The aim is to "demonstrate the US military's ability to quickly deploy a large force to support NATO and respond to any crisis," he said.

"Our ability as an army to project power is absolutely fundamental to anything that we would get done," he said.

The increasing military operations and naval exercises reflect the new momentum President Donald Trump’s administration has given to the US policy of "strategic rivalry" with China and Russia, two countries seen increasingly as a threat by Washington.

Defense Secretary Mark Esper said Saturday the Pentagon would shift focus away from the Middle East and toward countering what he characterized as "revisionist powers" of Russia and China.

Esper made the comment as he was outlining the Pentagon's strategic goals and priorities at an annual forum at the Ronald Reagan Presidential Library in Simi Valley, California.

During his speech, the Pentagon chief said he was sticking to the national defense priorities set by his predecessor, Jim Mattis, and accused Moscow and Beijing of seeking "veto power" over the economic and security decisions of smaller countries.

Esper also said he appreciated the recent military budget that would allow the Pentagon to address modernization and readiness shortfalls in the face of growing challenges from Russia and China.