"The government of Canada has concerns about how section 526 of the December 2007 US Energy Independence and Security Act could be interpreted to include Canadian oil sands," Eugenie Cormier-Lassonde, a spokeswoman for Canada's foreign affairs department, told AFP.
Section 526 of the law prohibits the US government from procuring alternative fuels with higher lifecycle greenhouse gas emissions than conventional petroleum sources.
Canada's Alberta oil sands have been classified as an unconventional energy source and, according to environmentalists, they create up to five times more carbon emissions than conventional oil production.
While conventional crude oil is pumped from the ground, oil sands must be mined and bitumen separated from the sand and water, then upgraded and refined.
"Classifying fuel from the oil sands as non-conventional fuel under section 526 would unnecessarily complicate the integrated Canada-US energy relationship," said Cormier-Lassonde.
"This would in turn have unintended consequences for both countries," she said.
The Bush administration has encouraged oil sands development to lessen US dependence on Mideast oil imports.
But all three US presidential candidates hoping to replace Bush are proponents of tackling global warming, which officials fear could lead to a narrow interpretation of the act.
At an estimated 173 billion barrels, Alberta's oil sands are the second largest oil reserve in the world behind Saudi Arabia, but they have been neglected, except by local companies, because of high extraction costs.
Since 2000, skyrocketing crude oil prices and improved extraction methods have made exploitation more economical, and have lured several multinational oil companies to mine the sands.