Donald Trump has reportedly sent sharply worded letters to several Nato member countries, urging them to spend more on their self-defence, in an escalation of the US president’s long-standing feud with the military alliance.

In letters reportedly sent to leaders of Nato countries like Canada, Germany, Spain, and the Netherlands, Mr Trump accused the allies of “underspending” on their national defence, and warned that the US may soon change its stance on the alliance.

“The United States continues to devote more resources to the defence of Europe when the Continent’s economy, including Germany’s, are doing well and security challenges abound. This is no longer sustainable for us,” Mr Trump wrote in a letter to German Chancellor Angela Merkel, according to the New York Times.

A National Security Council spokesperson declined to comment on the record, but told The Independent on background that Mr Trump remained committed to the alliance. He added that the best way to signal Nato's resolve would be for every ally to "share their burden of our collective defence".

Mr Trump has long been critical of the 29-member alliance, claiming the US has been “taken advantage of” by countries that do not want to pay their fair share. But the letters show Mr Trump has doubled down on this position going into the Nato summit next week – the first major summit since the contentious G7 meeting last month.

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The Nato allies agreed in 2014 to spend more on their own national defence, aiming to eventually contribute two per cent of their GDP to the effort. Many members countries say they are still working towards that goal, and are frustrated by Mr Trump’s threats.

Norwegian Defence Minister Frank Bakke-Jensen told the Associated Press that his country is ”following up” on the 2014 agreement, adding that Norway has spent “far beyond” Nato’s target for spending on new military equipment.

Belgian Prime Minister Charles Michel, meanwhile, told reporters last week he was “not very impressed” by Mr Trump’s letter.

“Belgium has halted the systematic fall in defence spending and takes part in a lot of military operations,” he said, according to German broadcaster Deutsche Welle.

Donald Trump singles out Germany on NATO defence contributions

Germany announced earlier this month that it would increase its defence spending to one-and-a-half per cent of its GDP by 2024, prompting US Defence Secretary James Mattis to say they were “on the right track”.

But Mr Trump criticised Ms Merkel in his letter last week, saying it was difficult to justify 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,” according to the Times.

The letters come after Mr Trump sparred with members of the Group of 7 at a summit last month, refusing to sign a joint communique and calling Canadian Prime Minister Justin Trudeau “dishonest and weak”. Many of those same leaders with be present at the Nato summit in Brussels next week, after which Mr Trump will meet with Russian President Vladimir Putin – a major NATO foe.

In an interview with CBS last week, national security adviser John Bolton attempted to steer the conversation away from the Russia summit, saying: “If you think Russia’s a threat, ask yourself this question: Why is Germany spending less than 1.2 per cent of its GNP?”