US Defence Secretary Pete Hegseth has lit into NATO allies, announcing a six-month Pentagon review of American forces in Europe that will hinge on how quickly they take charge of their own defence.

“This will be a real review. It's designed to ensure that NATO is moving fast and irreversibly towards Europe leading, stepping up to take primary responsibility for the defence of Europe,” he told his NATO counterparts in Brussels on Thursday.

Hegseth didn't stop there. He lambasted European allies for refusing to give US forces access to bases in Europe to launch attacks on Iran, calling it “shameful”.

“These allies, they put America’s sons and daughters, our sons and daughters, at risk by denying them the predictable access, basing and overflight that never should have been in question at all,” he said.

The Pentagon chief called for a reboot of the 32-nation alliance into what he dubbed “NATO 3.0” – a hard-line military alliance capable of deterring any threat. “NATO 3.0 is post-Cold War recognition that [NATO] needs to go back to a real hard-line military alliance that has real military capabilities capable of deterring right here on the continent and taking the lead for the conventional defence of Europe,” he told reporters.

As part of that push, Hegseth said the United States will invest $US1.5 trillion ($2.1 trillion) in its own defence in 2027, building what he called an “arsenal of freedom” that “first and foremost protects America and American interests but also backstops the strength of NATO and our allies”.

His remarks come weeks after the US told allies it would no longer supply certain warships and aircraft if one of them came under attack. On June 3, the US signalled it would withhold an aircraft carrier, support ships, aerial refuelling planes, and dozens of fighter jets in a crisis. NATO’s supreme allied commander, an American, is now working on back-up plans to defend Europe without those assets.

The Trump administration insists it needs to plan for two simultaneous conflicts and wants more military resources available should a conflict break out with China in the Indo-Pacific.

Under Article 5 of NATO’s founding treaty, all 32 allies pledge that an attack on one is an attack on all. But the article doesn't oblige them to provide military support. In essence, the US is scaling back how it might help if an ally triggers Article 5. The US has by far NATO’s biggest armed forces, though it doesn't intend to withdraw its nuclear weapons in Europe, which are key to NATO’s deterrence.

Hegseth said he told US allies they “have to be willing to stand up and do something in a strong way about” the defence of their own continent. European allies and Canada are now scrambling to work out how to plug the gaps left by the US pullback.