TORONTO, Jan. 26 -- Stephen Harper, elected Monday as prime minister, warned the United States on Thursday to back off from its challenge of Canadian sovereignty in Arctic waters that are fast thawing from global warming.

In the first news conference since his election, Harper upbraided the U.S. ambassador for asserting that the icy polar regions, including the legendary Northwest Passage, are international waters.

Canada claims that its archipelago of some 16,000 islands makes that region Canadian territory.

"The United States defends its sovereignty. The Canadian government will defend our sovereignty," Harper said. "It's the Canadian people we get our mandate from, not the ambassador of the United States."

The two countries -- as well as Russia -- have had conflicting claims in the Arctic for at least three decades. Harper's Conservative Party has proposed expanding Canada's military presence, building new icebreakers and creating an early warning system to detect other ships, all to enforce its claim of sovereignty.

The Liberal government Harper will replace has made similar claims and proposals. But it was unexpected that Harper would launch a sharp verbal attack against the United States on this subject so quickly, and he extended a press conference announcing his Feb. 6 inauguration to bring it up.

"The changing ice conditions are driving this issue to the top of the political agenda," said Michael Byers, an international law expert at the University of British Columbia.

"We've essentially been able to avoid problems over this in the past because the ice has been too thick and too hard to make it a commercially viable route," Byers said. "But, of course, the ice is melting."

He said that "within the next decade or two, there will be a major international shipping route around the northern side of North America." That will shorten the route for Europe-Asia shippers by 4,000 miles, Byers said, adding that it also presents security concerns as a "backdoor to North America that is wide open."

David Wilkins, the U.S. ambassador to Canada, was quoted Wednesday at a discussion in Ontario as saying, "We don't recognize Canada's claims to those waters."