As Prime Minister Justin Trudeau prepares to attend the upcoming NATO summit in Brussels, U.S. President Donald Trump is calling on Canada to meet the alliance's defence spending targets.

In the June 19 letter, Trump says there is "growing frustration" in the United States with North Atlantic Treaty Organization allies like Canada that have not increased defence spending as promised.

"This frustration is not confined to our executive branch. The United States Congress has taken note and is concerned as well," reads the letter, first reported by iPolitics. The Canadian Press has confirmed the authenticity of the letter.

"The United States is increasingly unwilling to ignore this Alliance's failure to meet shared security challenges," Trump writes.

The letter comes as tensions between Canada and the United States have risen to a dramatic high, with NAFTA talks all but stalled and a dispute over steel and aluminum tariffs imposed by the United States that has led Canada and other countries to counter with a long list of retaliatory tariffs.

Trump also had harsh words for Trudeau following the G7 meetings in Quebec earlier this month, calling the prime minister "dishonest and weak" after backing the U.S. out of the final joint communique issued by leaders.

The Liberals promised last year to increase spending on the military by 70 per cent over the next 10 years. But even with this increase, Canada will fall short of NATO's target of spending two per cent of GDP on defence.

Looking ahead to the coming NATO meeting next month, Trump says in his letter to Trudeau he appreciates Canada's defence contributions around the world, including in Iraq, Syria and Afghanistan, and on United Nations peacekeeping missions.

But these contributions "do not excuse any of us from our commitments to ensure NATO has the resources it needs," he writes.

Defence is 'rigorously costed'

"As one of our most capable allies and a leader in worldwide security, Canada's continued defence spending of less than two per cent undermines the security of the alliance and provides validation for other allies that also are not meeting their defence spending commitments," the letter says.

"At the summit, we must ensure alliance credibility by living up to our agreed commitments. I expect to see a strong recommitment by Canada to meet the goals to which we have all agreed in the defence investment pledge."

Renee Filiatrault, a spokeswoman for Defence Minister Harjit Sajjan, said Friday the Liberal government's planned spending increase has been "rigorously costed, is fully funded, and serves Canada's defence needs. It also upholds our long-standing role as an active contributor to global peace and security."

"Canada is proud to have contributed to every NATO operation since the founding of the alliance more than six decades ago. Canada's participation in NATO operations around the world is a tangible signal of our commitment to the trans-Atlantic alliance."