The president would need to rely on Ottawa to track Chinese and Russian missiles that could come at the U.S. from over the horizon.

Some experts are worried about the cost and timeline of U.S. President Donald Trump's proposed "Golden Dome" missile defence system, which Canada has expressed interest in joining. They also say it could set off a space-based arms race that would escalate global tensions.

Prime Minister Mark Carney declined to say how much Canada might spend if it decided to take part in the Dome project.