Donald Trump has dodged a question about whether he is still committed to Article 5 of NATO, the alliance's founding principle.
The clause asserts the principle of collective and mutual defence, meaning an attack against one NATO ally is considered an attack against all members.
It came as the US president spoke to reporters on Air Force One on his way to the NATO summit in The Hague, the Netherlands, and was asked about his commitment to Article 5.
He replied that it "depends on your definition" of it, while adding that he was "committed to being their friend".
Mr Trump added: "You know, I've become friends with many of those leaders, and I'm committed to helping them.
"I'm committed to saving lives. I'm committed to life and safety, and I'm going to give you an exact definition. When I get there. I just don't want to do it on the back of an airplane."
The NATO summit is intended to signal to Russian President Vladimir Putin that the alliance is united and set on expanding and upgrading its defences to deter any attack on its members from Moscow.
But Mr Trump's comments cast renewed doubt on his willingness to protect the US's allies.
However, NATO Secretary General Mark Rutte said he had "no doubt" that the US was "totally committed" to the mutual defence clause and NATO.
It comes after Volodymyr Zelenskyy warned that Russia could attack a NATO country within five years to test the alliance.
The Ukrainian president made the comments in an interview with Sky News' chief presenter Mark Austin.
But when asked if Russia could attack within months, Mr Zelenskyy said he did not "believe [Putin] is ready".
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Mr Zelenskyy also said plans for NATO members to increase defence spending to 5% of GDP by 2035 were "very slow" - adding: "We believe that, starting from 2030, Putin can have significantly greater capabilities.
"Today, Ukraine is holding him up, he has no time to drill the army."
Prime Minister Sir Keir Starmer, Mr Trump and the other leaders of NATO's 32 member states are expected to approve the target when they meet at the summit in The Hague.
As Sir Keir pledged to boost the UK's defence and security spending to meet the goal, he told reporters on his way to the summit that it was "pretty obvious we're living in volatile times, probably more volatile than most of us have lived through recently".
His government warned in a new National Security Strategy that it must "actively prepare" for the UK "coming under direct threat, potentially in a wartime scenario" for the first time in many years.
(c) Sky News 2025: Donald Trump dodges commitment to founding NATO principle