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

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.

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.