Prime Minister Stephen Harper has postponed the North American leaders' summit with U.S. President Barack Obama and Mexican President Enrique Pena Nieto at a time when relations with both leaders are chilly.

The unexpected move allows Mr. Harper to avoid an awkward side-by-side news conference with Mr. Obama at a February summit that all three governments were expecting would be dominated by the proposed Keystone XL oil pipeline – now at the top of the political agenda in Washington.

Instead, the Prime Minister surprised the United States and Mexico by postponing the event, popularly dubbed the Three Amigos summit, to an unspecified date late in the year. "We intend to proceed with the meeting later in 2015," said Mr. Harper's communications director, Jason MacDonald.

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Mr. Harper's government did not provide an explanation to either country, instead noting that they had never officially confirmed the date in February, sources said.

But planning was advanced for a late February summit, with the dates set aside on the always-packed calendars of the U.S. President and his Mexican counterpart.

The Canadian Council of Chief Executive Officers, which has long been involved in trilateral business issues, produced recommendations for the leaders last month, noting the summit was scheduled for February.

The lack of explanation for the late change led some U.S. and Mexican officials to ascribe it to Canadian concern about the risks of an awkward encounter with two leaders who are at odds with Mr. Harper over Keystone and Canadian visa requirements on Mexicans. In Ottawa, it is said summit agenda items had not been nailed down, a hint at differences between governments.

Facing an election in October, Mr. Harper has in recent months emphasized his prime ministerial stature on the world stage – using summits to call for action against Islamic State or to blast Russian President Vladimir Putin for interference in Ukraine. But hosting a summit likely to highlight disputes with powerful neighbours would be a political risk as he prepares to go to the polls.

Putting off the meeting might also heighten speculation that Mr. Harper will call a spring election – speculation Mr. MacDonald denies. "There will not be an election in the spring," he said.

Traditionally, Canadian governments have wanted to ensure regular North American summits, in part because they mean direct face time with the U.S. President.

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But Mr. Harper's relations with his North American counterparts are unusually prickly at the moment.

Mexico has expressed displeasure that Canada continues to require its citizens to obtain visas before visiting.

Mr. Pena Nieto publicly raised the issue at last February's summit in Toluca, Mexico, but his government is increasingly frustrated the requirement remains even though changes to Canada's asylum system dramatically reduced refugee claims by Mexicans – the reason Canada cited for imposing visas. In Mexico, it is the touchstone political issue in Canadian relations, and Mr. Pena Nieto was expected to bring it up again in February.

The chill with Mr. Obama has included differences over items such as a new Windsor-Detroit bridge, but the U.S. delays in deciding whether to approve Keystone XL top the grievance list. At last year's summit, Mr. Harper pressed the U.S. President on Keystone, and Mr. Obama rebuffed him and expressed the need for both countries to reduce greenhouse gas emissions.

The Keystone project is an even bigger political issue now. Republicans have control of both houses in Congress and are expected to pass a bill approving the oil pipeline as early as next week. Mr. Obama has threatened to veto it.

Attending a summit now would probably force him to defend a veto, and his criticisms of the proposed pipeline have grown more pointed. Mr. Harper's government has taken to describing the Keystone debate in the United States as an argument between the President and the American people.