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Mr Trump pledged the United States support for the mutual defence of NATO members, casting aside concerns that his failure to mention the commitment had weakened the alliance.

He said: I'm committing the United States to Article 5.

"Certainly we are there to protect, and that's one of the reasons that I want people to make sure we have a very, very strong force by paying the kind of money necessary to have that force.

But yes, absolutely, I'd be committed to Article 5."

Mr Trump made his comments while speaking to reporters at a news conference on Friday with visiting Romanian President Klaus Iohannis.

Article 5 states that an attack on one member is an attack on all members and binds the allies to come to that country's defence.

But, during his trip to Europe to meet NATO members, Mr Trump spent more time berating them for their lack of contributions than committing to Article 5.

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It left the heads of European countries deeply concerned at his lack of commitment and the fact that he didnt mention the clause in a speech at NATO headquarters in Brussels.

Instead, in that speech, Mr Trump demanded allies live up to a pledge to spend two per cent of their gross domestic product on defence by 2024.

There are rumours he had wanted to demand three per cent instead from member states, but had backtracked on that idea.

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He did not specifically mention Article 5, which has only been invoked once, after the September 11, 2001, terrorist attacks.

However, The White House later reaffirmed the commitment for him in a statement announcing that the president would visit Poland next month as part of his second foreign trip.

They billed it as showing America's support of Poland, as well as the president's commitment to strengthening NATO's "collective defence."

Mr Trump's omission in Brussels raised concerns on both sides of the Atlantic.

But White House aides said the president's support was implied even though he deliberately did not utter the words.

Still allies had questions about Trump's belief in the value of NATO, which he had termed "obsolete" during the presidential campaign.

On Friday, Trump noted that only a handful of NATO's 29 members - Montenegro joined just this week - were meeting the two per cent pledge. But he said the US would abide by its treaty obligations.

He said: "We're going to make NATO very strong.

"You need the money to make it strong. You can't just do what we've been doing in the past."

More:

Trump backs NATO defence pledge but again demands nations pay up - Express.co.uk