President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE reportedly wrote letters to a number of leaders of NATO-member nations, criticizing them for failing to live up to their spending obligations toward the international alliance.

The New York Times reported Monday that Trump wrote last month to German Chancellor Angela Merkel, Canadian Prime Minister Justin Trudeau and leaders in Belgium and Norway. The letters mark an escalation of Trump’s frequent critique that the U.S. shoulders too much of the burden in funding NATO.

Trump reportedly chastised Merkel for the country’s lack of NATO spending, saying it “provides validation for other allies that also do not plan to meet their military spending commitments, because others see you as a role model.”

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NATO members agreed in 2014 to move toward spending at least 2 percent of their respective gross domestic products on defense by 2024.

Trump also warned allies that the U.S. would reconsider its troop placement if nothing changes.

“It will, however, become increasingly difficult to justify to American citizens why some countries do not share NATO’s collective security burden while American soldiers continue to sacrifice their lives overseas or come home gravely wounded,” Trump reportedly wrote to Merkel.

The report comes one week before Trump is set to depart for the NATO summit in Brussels.

Trump has repeatedly complained since taking office that NATO allies take advantage of the U.S. by not paying their fair share to fund the alliance. He has also suggested the U.S. is no longer interested in helping police the world, and would rather focus on domestic priorities.

Trump roiled allies during last year's summit when he did not vocally reaffirm the United States's commitment to mutual defense, which is part of the NATO charter.

National security adviser John Bolton said Sunday that Trump "wants a strong NATO," but echoed the president's calls for other nations to contribute more.

"When people talk about undermining the NATO alliance, you should look at those who are carrying out steps that make NATO less effective militarily," Bolton said on CBS's "Face the Nation."