U.S. President Donald Trump cast doubt on Thursday on his willingness to defend Washington's NATO allies, saying he would not do so if they are not paying enough for their own defense.

The president has discussed possibly favoring members of the alliance that spend a set percentage of their GDP on defense, sources told NBC News.

President Donald Trump told reporters Thursday the U.S. would not defend North Atlantic Treaty Organization allies from a military attack if they are behind on defense spending encouraged by the organization,

Trump has reportedly discussed the possibility of adjusting engagement to favor allies that spend a set amount of GDP on defense.

He threatened not to support others if they haven’t “paid” enough, but did not seem to know that the only time the alliance was triggered was to help the U.S. after 9/11.

US President Donald Trump issued a strict warning to NATO members on Thursday, calling for increased spending. He also casted doubt on translantic alliance' support for US in times of crisis.

President Trump is considering not defending NATO members that do not meet the alliance's defense spending threshold, two senior administration officials told NBC News

Donald Trump's nominee to be U.S. ambassador to NATO, Matthew Whitaker, pledged on Tuesday to strengthen NATO and said the Republican president remains committed to the alliance. "If confirmed, I will work tirelessly to strengthen the alliance,

President Donald Trump’s pick as NATO ambassador is reassuring senators at his confirmation hearing that the Trump administration’s commitment to the military alliance is “ironclad.”