Delays in US approval of the controversial Keystone XL pipeline to carry oil from Alberta has forced Canada to consider a "different track" that could see the oil shipped to China instead, Prime Minister Stephen Harper said.

"I am very serious about selling our oil off this continent, selling our energy products off to Asia. I think we have to do that," Harper told broadcaster CTV in a year-end interview.

"When I was down in the United States recently it was interesting, I ran into several senior Americans who all said 'Don't worry we'll get Keystone done. You can sell all of your oil to us.' I said, 'Yeah we'd love to but I think the problem is now that we're on a different track."

Supporters say the plan to bring oil from Canada's tar sands to the United States is the ultimate shovel-ready job creation project and would spur the hiring of thousands of workers.

Environmental activists fear an accident along the 1,700-mile (2,700-kilometer) pipeline extension would be potentially disastrous for aquifers in central US Great Plains states.

Others oppose the multibillion-dollar project because exploiting the tar sands requires energy that generates a large volume of greenhouse gases that scientists blame for global warming.

The Obama administration has ordered an extra environmental assessment of a possible new route through Nebraska, which could delay a final decision until after next November's election.

That move prompted Obama's opponents to accuse him of dodging a difficult issue to avoid angering sections of his Democratic political base vote.