U.S. President Donald Trump's ambassador to Canada is warning of consequences to the continental defence pact if Canada does not move forward with the purchase of 88 F-35 fighter jets.

"NORAD would have to be altered," U.S. Ambassador to Canada Pete Hoekstra told CBC News in an exclusive interview at Luke Air Force Base in Arizona.

He says the United States would likely need to purchase more of the advanced fighter aircraft for its own air force, and would fly them more often into Canadian airspace to address threats approaching the U.S.

"If Canada is no longer going to provide that [capability], then we have to fill those gaps," said Hoekstra.