Since taking office in 2017, Donald Trump has on a multitude of occasions slammed NATO member states for failing to meet their annual defence spending obligations and insisted on fair burden-sharing.

Over the course of 2018, US President Donald Trump repeatedly expressed a willingness to withdraw from NATO, The New York Times reported, citing current and former senior administration officials.

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Although the unnamed sources said that they were not sure if Trump was serious, they allegedly feared that POTUS would return to his threat as other alliance member states failed to boost their military donations to NATO and reach the spending target set by the bloc.

Days ahead of a NATO summit in Brussels last summer, Trump purportedly questioned the alliance's raison d'être while speaking to senior national security officials, describing it as an exhausting burden on the United States.

The New York Times report suggests that Trump complained about Europe's failure to meet defence spending goals, thus leaving the US to "carry an outsize burden".

POTUS was allegedly frustrated with the fact that his transatlantic allies would not, on the spot, pledge to donate more. But at another leaders meeting during the same summit, NATO Secretary-General Jens Stoltenberg praised Washington's example and suggested that European member states follow suit — Trump was allegedly taken by surprise.

Trump was purportedly annoyed, in particular, with German Chancellor Angela Merkel and her country's military spending of 1 percent of its GDP.

At the time, then-Defence Secretary Jim Mattis and White House National Security Adviser John Bolton struggled to stick to American strategy without mentioning the potential withdrawal that would inevitably undermine Washington's influence in Europe and embolden Russia, the newspaper wrote.

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According to The New York Times, national security advisers are increasingly concerned over a possible pullout from NATO, as well as Trump's purported efforts to keep his encounters with Russian President Vladimir Putin secret from his own aides, and an ongoing investigation into the alleged collusion between the Trump campaign and the Kremlin.

"It would destroy 70-plus years of painstaking work across multiple administrations, Republican and Democratic, to create perhaps the most powerful and advantageous alliance in history. And it would be the wildest success that Vladimir Putin could dream of", Michèle A. Flournoy, an under-secretary of defence under President Barack Obama, told the media outlet.

The newspaper further cited retired Adm. Gen. James G. Stavridis, the former supreme allied commander of NATO, who said that "even discussing the idea of leaving NATO — let alone actually doing so — would be the gift of the century for Putin".

After The New York Times reached the White House for comment, a senior administration official cited Trump's remarks in July 2018, when he described Washington's commitment to the military alliance as "very strong", with the bloc itself being "very important".

The insiders, who spoke to the newspaper on condition of anonymity, assumed that with a weakened NATO, President Putin would have "more freedom to behave as he wishes", thus setting up Russia as a "counterweight" to the United States and Europe.

Although President Trump has not publicly threatened to leave the transatlantic alliance, relations between the US and Europe have hit their lowest point since he blasted other NATO members for not complying with their obligations to boost defence spending.

Trump has on numerous occasions emphasised that the other members of the bloc should pay their "fair share" and stressed that only five of the 29 member states were spending two percent of their GDP to defence, which was “insufficient to close gaps in modernising, readiness and the size of forces".

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On the sidelines of a NATO summit in Brussels in July, the allies agreed to start spending two percent of their GDP by 2024, with Trump pointing out that he was convinced that they would increase defence expenditures in line with their commitments.

At the same time, the US president suggested raising the military spending commitment up to four percent of GDP – that proposal, however, failed to find support.