In a repositioning that could start within weeks, the Pentagon will be sending home about 6,400 of its military personnel in Germany, and move nearly 5,600 to other NATO countries.

That will take the total US presence in Germany to about 24,000, Esper said.

A key aim of the rotation is to reinforce NATO's southeastern flank near the Black Sea, he said.

Another, which involves moving US command structures to Belgium, is to improve coordination with the NATO command.

Some could also go to Poland and the Baltic states if those countries reach final agreements with Washington on the idea, Esper said.

"These changes will unquestionably achieve the core principles of enhancing US and NATO deterrence of Russia; strengthening NATO; reassuring allies; and, improving US strategic flexibility," Esper said.

The move could have a significant economic and strategic impact in Germany, where tens of thousands of US troops have been stationed since the end of World War II.

Earlier this month the leaders of four German states urged the US Congress to block the troop reduction, warning it could weaken the Atlantic alliance's deterrence against Moscow.

Esper said the move was long-discussed and was not the result of President Donald Trump's unhappiness with the relationship between Washington and Berlin.

He said the relocation of troops and a fighter jet squadron to Italy, and more rotations of armored Stryker units into the Black Sea region, was focused on the potential threat to southeastern Europe from Russia, whose military ambitions were underscored by the 2014 annexation of the Crimea.

The goal is "to enhance deterrence and reassure allies along NATO's southeastern flank," Esper said.