The US Army in Europe and Africa said an infantry brigade combat team and an airborne division were to redeploy to their Kentucky home base without replacement, after Romania previewed the scale-down.

"This is not an American withdrawal from Europe or a signal of lessened commitment to NATO and Article 5," it said in a statement from its German headquarters, referring to the alliance's collective defence principle.

"Rather this is a positive sign of increased European capability and responsibility."

The US had been expected to order drawdowns in Europe following a review of its military deployments worldwide. But it was beaten to an official announcement by Romania.

Bucharest's defence ministry said Wednesday that the US was to halt the rotation of a brigade that had elements in several NATO countries, including Bulgaria, Romania, Slovakia and Hungary.

But NATO swiftly played down the planned reduction in personnel amid fears of a US pullback at a time where Europe contends with an aggressive Russia.

"This force posture adjustment will not change the security environment in Europe," the US military added.

US President Donald Trump has repeatedly criticised NATO, and insisted that European allies boost military spending as Ukraine battles Russia's invasion.

The prospect of a US pullout is fraying the nerves of allies, especially given fears that Russia could look to attack a NATO country within the next few years if the war in Ukraine dies down.