(CNN) 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.

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'Growing frustration'

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.

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