Washington DC - President Donald Trump on Thursday renewed doubts over his commitment to the NATO alliance, saying countries that aren't spending adequately on their militaries don't deserve defense.

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

Trump and others in his administration have frequently questioned whether the US – 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 leader, 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 US.

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"They should be paying more," he said.

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

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

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