"If they don't pay, I'm not going to defend them," he told reporters in the Oval Office.

Trump has frequently questioned whether the United States -- by far the biggest military in the transatlantic alliance and ultimate guarantor of Europe's security since World War II -- should continue its central role in NATO.

The Republican, who began his second term in January, doubled down on his criticism that some NATO members do not spend enough on their defense budgets and overly rely on the United States.

"They should be paying more," he said.

Trump was responding to reporters after NBC News reported earlier Thursday that he is considering a plan to calibrate US military support in a way that favors member countries which spend a higher proportion of more of their GDP on defense.

The president has previously called for allies to lift annual defense spending to five percent of GDP from the current two-percent target, which NATO expected only 23 of 32 members to meet last year.

Countries deemed to be underspending might not be defended if attacked, according to the reported plan.

The move would weaken NATO's core Article 5 which stipulates that any member attacked will be defended by all the others.

Trump also questioned whether allies, including France, would defend the United States.

"If the United States was in trouble and we called them. We said, 'We got a problem, France. We got a problem. A couple of others, I won't mention. Do you think they're gonna come and protect us? Hmm. They're supposed to. I'm not so sure," Trump said.

French President Emmanuel Macron, however, said France was a "loyal and steadfast ally."

"We have always been there for each other," Macron told reporters in Brussels after a meeting of EU leaders, where they agreed to strengthen Europe's defense.

Macron said France had shown "respect and friendship" to the United States, and "we are entitled to ask for the same thing."

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