"I'd like to emphasise this decision does not impact the executability of our regional plans," US General Alexus Grynkewich, NATO's Supreme Allied Commander Europe, told journalists.

"As allies build up their capability, the United States is able to pull capability back and use it for other global priorities, so I'm very comfortable with where we are," he added.

The sudden move from Washington followed a spat between Trump and German Chancellor Friedrich Merz over the war in Iran and caught allies unaware.

But the Trump administration had long been telling European countries that the United States was looking to pull out forces as it focuses on other threats around the world.

"We should expect there to be a redeployment of US forces over time as allies build their capacity," Grynkewich said.

"I can't really give you an exact timeline, it's going to be an ongoing process for several years."

While European NATO members have downplayed the impact of the US withdrawal, the abrupt manner of the announcement has reinforced worries over Trump's commitment to the alliance.

The US president has lashed out at Europe over its response to his war with Iran and threatened repeatedly that he could consider quitting NATO.

Europe will seek to smooth over the bad blood at a meeting of NATO foreign ministers in Sweden later this week that will include US Secretary of State Marco Rubio.

The alliance is gearing up for a summit with Trump in Turkey in July at which it wants to showcase Europe's increase in defence spending to keep the US leader happy.