US Defense Secretary Pete Hegseth has severely criticised some of America's Nato allies, while announcing a six-month review of the presence of US forces in Europe.
"It's a review that some countries will fail, and others will pass with flying colours", he said at a meeting of Nato defence ministers in Brussels, singling out allies that he suggested had been "free-riding".
Hegseth was also highly critical of Nato member states that had imposed limits on help to US forces during the war with Iran.
His announcement follows a US decision to scale back its commitments to a high readiness force within the alliance known as the Nato Force Model (NFM).
The aim of the review, which Hegseth termed Nato 3.0, was to "ensure that Nato is moving fast and irreversibly toward Europe leading" on security on the continent.
The US wants Nato members to contribute more to defence spending in Europe and says some countries have not shown how they will reach an agreed target of 5% of national economic output (GDP) which includes 3.5% on core defence and 1.5% on related infrastructure.
Nato Secretary-General Mark Rutte said defence spending had already gone up €90bn (£78bn; $103bn) last year - a rise of almost 20% - and Europeans were "already backfilling" resources that the US was cutting back on.
Details of how the US will reduce its commitments have not been made public but it has been indicated they include air and naval capabilities.
Hegseth said Nato's annual dues would be "contingent on other countries meeting their defence spending targets; where other allies do not spend with urgency, our dues contributions will go down".
"Some of Nato's largest economies, some of our richest countries, allies that are happiest to go on about the rules-based international order and middle powers banding together, still seem to think the era of free-riding is here," he added.
He did not single out which countries he meant.
UK Defence Secretary Dan Jarvis attended the summit without a UK defence investment plan, after his predecessor John Healey resigned, warning that the plan fell "well short" of what was needed to protect the UK.
A senior Nato official conceded that "not everything" that the US was withdrawing "can be absolutely replaced" but Rutte said some work had already been done and further efforts were under way. He also revealed that the changes were taking effect immediately.
The Nato Force Model is a set of forces that the alliance's Supreme Allied Commander in Europe (Saceur) knows he "can count on" at short notice, the Nato official explained.
In May the US announced it would withdraw 5,000 troops from Germany after a row between President Donald Trump and German Chancellor Friedrich Merz over the war with Iran.
The same month Poland was told 4,000 troops would be pulled out only for Trump to later reverse the plan and promise 5,000 would be deployed.
Poland hosts up to 10,000 US troops on a rotational basis, and Defence Minister Władysław Kosiniak-Kamysz said on Thursday that the US was open to Warsaw's offer to provide a permanent base for US troops. He said a final decision would depend on the details of such an agreement.
Earlier, Trump threatened to halt all trade with Spain after the government in Madrid refused to allow the use of air bases on its territory for attacks on Iran. The US has two military bases there, Naval Station Rota and Morón Air Base.
Rutte said on Wednesday he expected members to show how they would deliver on the 5% promise.
He added: "Ahead of the summit in Ankara [on 7-8 July], allies will highlight how they're delivering on commitments made in The Hague last year. Investing 5% of GDP in defence by 2035. That's what we agreed.
"I expect nations to present clear, concrete and credible plans to reach that goal. Ideally, well ahead of the agreed timeline. Many are already showing that they are doing exactly that."
