The US intends to significantly reduce military contributions available to assist European allies in a crisis, including fighter jets, warships and mid-air refuelling aircraft, German news outlet Spiegel reported on Tuesday.

The NATO alliance is under unprecedented strain, with some European countries concerned that Washington may withdraw outright.

US President Donald Trump has slammed European allies for not spending enough on their militaries and pledged to withdraw thousands of troops from Germany. His ambition to take control of Greenland, a Danish overseas territory, has further inflamed transatlantic tensions.

Trump also fiercely criticised European allies for a lack of support in reopening the Strait of Hormuz for shipping amid the war on Iran, saying he was considering withdrawing from the NATO alliance and questioning whether Washington was bound to honour its mutual defence pact.

According to the Spiegel report, an envoy of US Defense Secretary Pete Hegseth briefed senior officials from member states on the plan at NATO headquarters in Brussels late last week.

Three sources familiar with the matter had told Reuters the Trump administration was planning to tell NATO allies last week it would shrink the pool of military capabilities available to the alliance during a crisis.

The US aims to provide only half the previous number of strategic bombers, the reports said.

Specifically, the number of US fighter jets is set to fall by a third, Spiegel cited U.S. envoy Alexander Velez-Green as saying during the closed-door meeting.

The U.S. Navy is also set to make fewer destroyers available to NATO, and the U.S. no longer intends to provide any submarines to the alliance.

Under the changes, Europe would be forced to provide its own reconnaissance drones, while the US plans to significantly scale back the provision of armed models.