The letter from the 23 senators called Canada a valued ally that has contributed to NATO operations around the world but added that the alliance "now faces one of the most severe threat landscapes in its history."

The letter urged all NATO allies and Canada specifically to uphold their NATO commitment and accelerate efforts to reach the 2% defense spending target.

Sent ahead of a NATO summit in July in Washington, the letter writers were led by Democratic Senator Jeanne Shaheen and Republican Senator Thom Tillis, the co-chairs of the U.S. Senate NATO Observers Group.

Spokespeople at the Canadian embassy in Washington and Trudeau's office in Ottawa did not immediately respond to requests for comment on the letter.

In April, Canada pledged billions more for the armed forces and said military spending was set to hit 1.76% of GDP by 2030, up from the current 1.4%. Officials told reporters then that there was no target date to hit the 2% mark.

The failure of many of NATO's 31 members to meet a defense spending target of at least 2% of gross domestic product has long been a source of tension with the United States, whose armed forces form the core of the alliance's military power. NATO estimates have shown that only 11 members are spending at the target level.