In a letter to Prime Minister Justin Trudeau dated June 19, the US president said there is "growing frustration in the US that key Allies like Canada have not stepped up defense spending as promised."

The text was made public by the website iPolitics, with a spokesman for the prime minister confirming its existence to AFP Saturday.

For Trump, the fact that Canada's defense spending represents less than two percent of GDP "undermines the security of the alliance," and "provides validation for other allies that also are not meeting their defense spending commitments."

Canada's defense spending was 1.3 percent of GDP in 2017, according to NATO.

In 2014, members of the treaty pledged to allocate two percent of GDP to defense. The US president, who has often lamented several member states' level of military spending, reiterated that he would demand that this commitment be honored at the NATO summit in Brussels on 11 and 12 July.

"It will become increasingly difficult to justify to American citizens why some countries continue to fail to meet our shared collective security commitments," he wrote.

A spokesperson for Canadian defense minister Harjit Sajjan said Saturday that the government committed in 2017 to increase military spending by 70 percent over 10 years -- and added that "Canada's participation in NATO operations around the world is a tangible signal of our commitment to the trans-Atlantic Alliance."

The letter comes amid tensions between Canada and the US, after the imposition of US tariffs on steel and aluminum -- and Trump's branding of Trudeau as "dishonest" and "weak" after the G7 summit.