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

Some 85,000 American troops are stationed in Europe, including 20,000 troops deployed to reinforce NATO's eastern flank after Russia invaded Ukraine in 2022, according to the US Defence Department.

The latest announcement came after reports this year that Washington could withdraw 10,000 troops from eastern Europe as the US focus shifts more towards the Indo-Pacific region, which the Pentagon called it's "priority theatre".

"The resizing of US forces is a result of the new priorities of the presidential administration, announced back in February," Romania's defence ministry said in a statement.

The decision "also took into account the fact that NATO has strengthened its presence and activity on the eastern flank," it added.

- 'Wrong signal' -

A NATO official told AFP that the body had been informed by the Trump administration in advance, and played down its significance.

"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.

Washington's commitment to the alliance remained "clear", the official added.

But experts such as George Scutaru, co-founder of the New Strategy Center, a Romanian think tank, warned the move was the "wrong signal" to send to Russia regarding the Black Sea region.

"Russia will consider that Black Sea is not so important for American interests in Europe," he told AFP.

Moscow "will be encouraged trying to put pressure more especially on Romania using drones, excursions in airspace," he added.

Scutaru said it was important for European allies to send "a signal of solidarity" and consider enhancing their "military presence to compensate," citing France in particular, on the eve of a visit by French Defence Minister Catherine Vautrin to Romania.

"1000 more, 1000 less, it does not influence a lot the military balance in the Black Sea. It's about a political signal," he said.

Poland's Defence Minister Wladyslaw Kosiniak-Kamysz told reporters that the country had "not received any information... about a reduction in the contingent in Poland".

Romania's Defence Minister Ionut Mosteanu however said the US decision was not a "withdrawal".

It was the "cessation of the rotation of a brigade that had elements in several NATO countries, including Bulgaria, Romania, Slovakia and Hungary", he told journalists.

- 'Unchanged' capabilities -

Mosteanu said that 900-1,000 US soldiers would remain in Romania.

They will help "to deter any threat" and represent "a guarantee of the United States' commitment to regional security", he said.

According to the latest government figures, about 1,700 US troops are currently deployed in Romania.

Mosteanu insisted that strategic capabilities were "unchanged". He added that a missile-defence system at the Deveselu NATO base "remains fully operational".

"The Campia Turzii air base continues to be a key point for air operations and allied cooperation, the Mihail Kogalniceanu base continues to be developed, and the American flag will remain present at all three sites," he added.

"An air-combat group will remain at the Kogzlniceanu airbase, as was the case before the outbreak of the conflict in Ukraine," he said.

The US decision will however "weaken the security" of Romania, which is a "frontline state", Phillips O'Brien, an analyst based at Scotland's University of St Andrews, warned.

"Please wake up, Europe -- the USA will not defend you against Russia," he wrote on X.