Washington's commitment to the alliance remained "clear" and its presence in Europe sizeable, a NATO official told AFP.

"Even with this adjustment, the US force posture in Europe remains larger than it has been for many years, with many more US forces on the continent than before 2022," the official said.

The US has been expected to announce drawdowns in Europe following a review of its military deployments worldwide -- but no move has been officially communicated yet.

Romania's defence ministry however previewed the scale-down Wednesday, saying Washington was to halt the rotation of a brigade that had elements in several NATO countries, including Bulgaria, Romania, Slovakia and Hungary.

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.

But NATO swiftly downplayed the significance of the planned US reduction in personnel.

"The US commitment to NATO is clear," the official said.

"NATO has robust defence plans in place and we are working to ensure we maintain the right forces and capabilities to deter potential aggression and provide for our collective defence."