A pipeline that would move heavy bitumen to the northern British Columbia coast from Alberta's oilsands carries unacceptable risks, Canadian and American environmental groups say.

The proposed route for Enbridge Inc.'s $5.5-billion pipeline passes through rivers and sensitive habitat, and a spill of heavy bitumen of the type it would carry would be devastating to the region, groups including Canada's Pembina Institute and the Natural Resources Defence Council in the United States say in a new report.

The report, titled Pipeline and Tanker Trouble, cites the engineering challenges of the project, which would link Bruderheim, Alta., near Edmonton, with Kitimat, B.C., aboriginal objections and the potential for massive environmental damage from super-tanker spills along the treacherous northern B.C. coast or pipeline spills along the route.

The report cites the example of the July 2010 Enbridge pipeline spill that eventually dumped three million litres of bitumen into the Kalamazoo River in Michigan. The cleanup, which had been expected to take two months, is now expected to stretch well into 2012, Susan Casey-Lefkowitz of the NRDC told CBC News.

"The [U.S.] Environmental Protection Agency is still cleaning that up and, basically, keep encountering problems and difficulties with the cleanup that they had not anticipated because, in part, they weren't prepared for the differences that cleaning up oilsands oil brings,"Casey-Lefkowitz said.

Because bitumen is much heavier than conventional oil, it can sink when it spills into water and mix with dirt along the bottom, which makes dredging much more difficult and time-consuming, the report says.

The groups claim Canadian pipeline regulations haven't been updated to account for the transport of bitumen. They also claim federal regulations would allow millions of litres of oil to be spilled before it could be detected.

Paul Stanway, communications manager for Enbridge Northern Gateway Pipelines, panned the report, which he said merely repeated criticism that has already been levelled against the pipeline project.

While the company is doing everything possible to mitigate potential problems, no energy project of this scale is without risk, Stanway told CBC News.

"You have to figure out where you are going to set the bar," Stanway said. "Are you prepared to take some minimal risk to provide energy around North America and, indeed, to markets around the world? Or are you prepared to take zero risk and not transport energy anywhere."

With files from The Canadian Press