The Canadian government attacked the U.S. Environmental Protection Agency on Wednesday over its assertion that the Keystone XL pipeline might worsen greenhouse-gas emissions.

In a letter signed by Ambassador Gary Doer, the Canadian government accused the EPA of "significant distortion and omission" in its assessment of TransCanada Corp.'s proposed pipeline from Alberta, across the United States, to the Gulf Coast.

Hours later, the U.S. House of Representatives overwhelmingly passed 270-152 the Keystone XL approval bill. The same bill passed 62-36 last month in the Senate, where the new Republican majority has tipped the balance in favour of Keystone XL, and sets the stage for a showdown with U.S. President Barack Obama, who has vowed to veto it. Keystone XL's backers – the Republican majorities in both the House and Senate along with some Democratic supporters – fall short of the two-thirds majority required to override a presidential veto.

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Republicans are already planning a second effort, tucking a Keystone XL approval bill into "must-pass" legislation, such as a spending bill, and thus setting up a higher-stakes confrontation with Mr. Obama, whose presidential veto might shut down the government.

"We're wasting our time today," said Representative Frank Pallone, a New Jersey Democrat and ranking member of the energy and commerce committee. He said it was the "11th time" the House had dealt with "a special deal for TransCanada's tar sands pipeline" and noted that the decision to issue a permit for border-crossing infrastructure was up to the President.

During the House debate, Keystone XL backers said it was time to end Mr. Obama's dithering and approve the project, first proposed six years ago, which, they said, would create tens of thousands of jobs and provide energy security.

Mr. Doer's letter was another shot in what has become an increasingly testy spat between Prime Minister Stephen Harper's government and Mr. Obama's administration over Keystone XL.

The President has vowed to veto any legislation stripping him of the authority to decide on Keystone XL.

House Speaker John Boehner, an Ohio Republican, accused Mr. Obama of turning his back on ordinary Americans. "Instead of listening to people, the President is standing with a bunch of left-fringe extremists and anarchists."

Mr. Doer's letter, delivered Tuesday to U.S. Secretary of State John Kerry, was made public Wednesday by the Canadian government. It said the EPA assessment, part of the multiagency assessment of Keystone XL, ignores a decade of Canadian achievement in cutting greenhouse gas emissions. The EPA is using 2005 figures from "two years before iPhones existed, completely neglecting the innovation and emissions reductions that have since occurred in the oil sands," Mr. Doer wrote.

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Meanwhile, TransCanada disputed the EPA's contention that Keystone XL will spur further development of the oil sands.

"We also reject the EPA's inference that at lower oil prices, Keystone XL will increase the rate of oil sands production and greenhouse gas emissions," Russ Girling, TransCanada's president and chief executive, said in a letter released Wednesday.

Mr. Obama has been increasingly negative about Keystone XL, suggesting it brings few, if any, benefits to U.S. consumers. "Understand what this project is: It is providing the ability of Canada to pump their oil, send it through our land down to the Gulf, where it will be sold everywhere else," he said last year, although he insists he has yet to make up his mind as to whether Keystone XL serves the U.S. national interest and won't worsen global warming, the criteria he has publicly laid out for making a final decision. TransCanada insists the oil from Keystone XL will not be exported.

Mr. Girling said neither low oil prices nor building Keystone XL will make any difference to continued development of Alberta's oil sands. "It is clear that building or not building Keystone XL will not cause production to go up or down, nor does the pipeline significantly exacerbate the problem of GHG [greenhouse gas] emissions," he said.