President Donald Trump sent letters to the leaders of NATO allies, including Germany, Belgium, Norway and Canada, demanding that they increase their defense spending and threatening to alter the US global military presence if they do not, according to a report in The New York Times on Monday.

The letters were sent last month, according to the Times, before a NATO summit in Brussels, Belgium, next week.

A source familiar with the letters confirmed their existence to CNN but did not confirm the language in them. A separate diplomatic source briefed on the letters described them as “very tough” and said they requested more defense spending and warned the US was losing patience.

According to the Times, many of the letters to the various leaders included similar language, but Trump’s letter to German Chancellor Angela Merkel was particularly critical.

“As we discussed during your visit in April, there is growing frustration in the United States that some allies have not stepped up as promised,” the letter to Merkel read, according to the Times, which cited a source who had seen it and sent excepts. “Continued German underspending on defense undermines the security of the alliance and provides validation for other allies that also do not plan to meet their military spending commitments, because others see you as a role model.”

Trump further hinted that the US could alter its global military presence if NATO allies do not increase spending on their own defense, according to the Times.

“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 wrote to Merkel, according to the Times.

The Times added that Trump told Merkel and other leaders, including Canadian Prime Minister Justin Trudeau and Norwegian Prime Minister Erna Solberg, that he is cognizant of domestic political pressures against increased military spending but that he has used “considerable political capital to increase our own military spending.”

The White House declined to comment on presidential correspondence, but a National Security Council spokesman told CNN, “The President is committed to the alliance, as he has stated repeatedly. The President has also been clear we expect our allies to shoulder their fair share of our common defense burden and to do more in areas that most affect them. There is no better way to signal NATO’s resolve than for every ally to allocate the resources necessary to share their burden of our collective defense.”

The letters fit a pattern of Trump critiques of NATO, particularly of defense spending by other members. NATO members committed to spend 2% of their gross domestic product on national defense at a 2014 summit in Wales, and Trump has repeatedly bemoaned that NATO allies have not fulfilled this commitment.

“Members of the alliance must finally contribute their fair share and meet their financial obligations,” Trump said during a May 2017 NATO meeting. “Twenty-three of the 28 member nations are still not paying what they should be paying and what they are supposed to be paying for their defense. This is not fair to the people and taxpayers of the United States.”

At the time, Trump did not reiterate the US commitment to NATO’s mutual defense provision, Article 5. He committed to Article 5 at a news conference that June, however.

Trump also previously referred to NATO as obsolete, although he later reversed that position.