Prime Minister Stephen Harper was again rebuffed in his bid to press U.S. President Barack Obama to approve the controversial Keystone XL pipeline when he raised the issue during a North American leaders' summit in Toluca, Mexico.

Mr. Harper discussed the matter with his U.S. counterpart during a one-on-one conversation on Wednesday afternoon before a formal summit with Mexican President Enrique Pena Nieto. Canadian officials have expressed frustration with Mr. Obama's delays on the matter and are concerned that a final decision could come too late for this year's construction season.

Mr. Obama said he has no plans to hasten the process for making a decision. "Keystone will proceed along the path that's already been set for it," he said during a press conference Wednesday evening.

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The proposed TransCanada Corp. pipeline, which would carry Alberta crude to the Gulf Coast, has become a source of tension for the two leaders. On Wednesday, Mr. Harper's cause was dealt another setback when a Nebraska court ruled that the governor's decision to allow the pipeline to pass through the state was unconstitutional.

Mr. Obama said he discussed a shared interest in addressing greenhouse gas emissions with Mr. Harper when the two leaders met earlier on Wednesday. And he said all decisions – and not just Keystone – should take greenhouse gas emissions into account.

"The science is irrefutable," Mr. Obama said. "We're already seeing severe weather patterns increase, and that has consequences for our businesses, for our jobs, for our families, for safety and security."

The three leaders smiled and waved during group photographs on Wednesday in Toluca's Cosmovitral botanical garden, where they were surrounded by floor-to-ceiling stained glass and leafy foliage. They talked for about two hours on Wednesday evening, and emerged with plans to increase co-operation on issues including education, travel, security and trade.

The leaders said they would expand trusted-traveller programs such as Nexus, which currently applies at the U.S. and Canadian border. The new plan, to be implemented this year, would provide for mutual recognition of Nexus and similar North American programs to ease travel for pre-approved individuals throughout the continent.

And during a brief bilateral meeting before the formal summit, Mr. Harper and Mr. Obama vowed to hold those responsible for ongoing violence in Ukraine accountable for their actions. They also said they welcomed reports of a truce between the government and the opposition as a step toward meaningful dialogue.

But despite the friendly gestures and promises to improve co-operation, there were clear signs that Mr. Harper's first official visit to Mexico has been strained over tensions about the Keystone approval and a Canadian decision to require visas from visiting Mexicans.

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The Mexican government is keen to see the requirement lifted, arguing it hurts businesses and tourism in Canada and prevents legitimate travellers from visiting. Mr. Harper made it clear this week that he has no plans to lift the requirement, calling it a sovereign decision that is not up for negotiation.

The Keystone pipeline would carry heavy crude from the oil sands in Western Canada to Nebraska, where it would connect with existing pipelines that would carry it the rest of the way to refineries on the Gulf Coast. The U.S. State Department recently issued a favourable assessment of the project, saying it would not substantially contribute to greenhouse gas emissions.

Mr. Harper said climate change is a shared concern, adding that Canada and the U.S. have similar targets for reducing emissions. "But in terms of climate change, I think the state department report ... was pretty definitive on that particular issue," he said on Wednesday.



He added that changes to streamline Canada's process for environmental review should also give investors more confidence, an apparent jab at the lengthy U.S. review process for Keystone.

The three leaders agreed to hold a North American energy ministers' meeting later this year to further co-operate on energy issues. Mexican officials have called for more collaboration on energy, saying their countries should use their strengths to help make North America a manufacturing powerhouse.

The leaders also discussed negotiations on the Trans-Pacific Partnership, which includes 12 countries in the Americas, Asia and the Pacific.