US President Donald Trump said he was strongly considering pulling the United States out of NATO after allies failed to back US military action against Iran, according to an interview with Britain's Daily Telegraph.

Trump described the alliance as a "paper tiger" and said removing the United States from the defense pact was now "beyond reconsideration," the newspaper reported. He said he had long held doubts about NATO's credibility.

"I was never swayed by NATO. I always knew they were a paper tiger, and Putin knows that too, by the way."

Trump, in a phone interview with Reuters, said one element of his speech scheduled for Wednesday evening would be to express his disgust with NATO for what he considers the alliance's lack of support for U.S. objectives in Iran.

He said he is "absolutely" considering withdrawing the United States from NATO, a treaty organization ratified by the US Senate in 1949.

NATO is a military alliance that ensures the security of the Euro-Atlantic area and is not designed to carry out operations in the Strait of Hormuz that would breach international law, France's junior army minister said on Wednesday.

"Let me remind you what NATO is. It is a military alliance concerned with the security of the Euro-Atlantic region. It is not designed to carry out operations in the Strait of Hormuz, which would be a breach of international law," Alice Rufo said at the War & Peace conference in Paris.

A German government spokesperson reaffirmed Berlin's commitment to the NATO defense alliance on Wednesday after comments by US President Donald Trump to Britain's Daily Telegraph, in which he said he had moved beyond reconsidering his country's membership.

"This isn't the first time he's done this, and since it's a recurring phenomenon, you can probably judge the consequences for yourself," said the spokesperson at a regular government press conference.

"It's not my place here to comment on the American president's words. I simply want to state on behalf of the German government that we are, of course, committed to NATO."

In Poland, Defense Minister Wladyslaw Kosiniak-Kamysz called for calm.

"I hope that amid the emotions surrounding the President of the United States today, a moment of calm will come," he said. "And why? Because there is no NATO without the United States, and it is in our interest that this calm comes. But there is also no American power without NATO."

NATO had no immediate comment.

British Prime Minister Keir Starmer said he would act in his country's interest, whatever the "noise." The instability caused by the Iran war meant Britain should pivot to focus on closer economic and defense ties with Europe, he said.

Finnish President Alexander Stubb told US President Donald Trump on Wednesday that "a more European NATO" was taking shape and that Europe was "shouldering responsibility," Stubb's office said in an emailed statement to Reuters.