TORONTO — The Trump administration is contemplating putting troops near its northern border with Canada, Prime Minister Justin Trudeau said Thursday — a move that could seriously strain relations between longtime allies said to share the world’s longest nonmilitarized border.

Trudeau said his government had “discussions” with the United States about the plan, which was first reported by Global News. The report said troops would be temporarily stationed within 18 miles of the Canadian border and flag people crossing at unofficial points of entry to border agents.

“Canada and the United States have the longest unmilitarized border in the world,” Trudeau said, “and it is very much in both of our interests for it to remain that way.”

“Canada is strongly opposed to this U.S. proposal,” Deputy Prime Minister Chrystia Freeland said, “and we have made that opposition very, very clear to our American counterparts, and we will continue to do so.”

Canadian officials declined to provide details on the U.S. proposal, which they first learned of a “few” days ago. Freeland said it would be “completely inappropriate” for her to comment on the internal discussions of the U.S. government.

Asked about the proposal at Thursday’s coronavirus briefing, President Trump said he would “find out about that.”

“I guess it’s equal justice to a certain extent,” he said. “In Canada, we do have troops along the border. We have a lot of things coming in from Canada. We have some illegal trade that we don’t like … And we don’t like steel coming through our border that’s been dumped in Canada so they could avoid the tariff.”

But earlier in the day, Freeland said the proposal was “not a trade issue." She stressed that a decision has not yet been made on the deployment.