Washington, Jul 3 (IANS): US President Donald Trump has written to leaders of several NATO countries, including Germany, Belgium, Norway and Canada, demanding that they increase their defence spending and warned that Washington "is losing patience with their failure to meet security obligations shared by the alliance".

According to a report in the New York Times on Monday, the letters were sent in June ahead of a NATO summit in Brussels next week.

Trump has repeatedly questioned the value of the alliance and claimed that its members were taking advantage of the US. CNN quoted a source as saying that the letters were "very tough".

The President has previously referred to NATO as "obsolete" although he later reversed that position.

The Times report said that 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 US 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 defence 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 indicated that Washington could alter its global military presence if NATO allies do not increase spending on their own defence.

"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.

It added that Trump told Merkel and other leaders, including Canadian Prime Minister Justin Trudeau and Norwegian leader Erna Solberg, that he was cognizant of domestic political pressures against increased military spending but that he used "considerable political capital to increase our own military spending".

The letters fit a pattern of the President's critiques of NATO, particularly of defence spending by other members. NATO members committed to spend 2 per cent of their gross domestic product on national defence at a 2014 summit in Wales.

Trump has repeatedly bemoaned that NATO allies didn't fulfil this commitment.