NATO Secretary General Jens Stoltenberg said Esper had assured anxious European partners that President Donald Trump's proposal to cut troops based in Germany by a third to 25,000 was not yet finalised.

The move has been criticised as weakening America's commitment to European defence as well as its ability to wield influence in the Middle East and Africa.

The plans caught Berlin and other allies by surprise, and Stoltenberg said Esper had used video talks on Wednesday to give reaasurances about future consultations.

"I welcome the fact that Secretary Esper in the meeting with NATO defence minsters was clear both on the US commitment (to defending Europe) but also that US will consult allies on the way forward," Stoltenberg told reporters afterwards.

US troops have been stationed in Germany since the end of World War II, but the resurgence of Russia's military ambitions under President Vladimir Putin has given the presence new importance.

Trump said he was cutting troop numbers to punish what he called "delinquent" Germany for not spending enough on its own defence, instead freeloading on the US.

Aside from strategic considerations, diplomats have voiced doubts about the cost effectiveness of Trump's plan, and Stoltenberg said Esper had been clear the matter was not yet finalised.

"No final decision has been made on how and when to implement the us intention," Stoltenberg said.