VANCOUVER - Environmentalists are ramping up their assault on the controversial Northern Gateway pipeline, arguing the $5.5-billion Enbridge (TSX:ENB) project poses an unacceptable risk to British Columbia's mountain rivers and pristine coastline.

A collection of groups including the Natural Resources Defense Council and the Pembina Institute released a report Tuesday that reiterates many of the criticisms they have already levied against the pipeline, claiming the heavy crude from Alberta's oil sands is especially dangerous and the proposed route too treacherous.

The report comes amid uncertainty surrounding TransCanada's (TSX:TRP) proposed Keystone XL pipeline that would run from Alberta to the U.S. Gulf Coast -- a reality that has made Enbridge's pipeline through northern British Columbia more attractive than ever.

Provincial and federal politicians have been especially vocal in their support for the Northern Gateway line, particularly in the face of questions about Keystone XL.

"It certainly will be a difficult challenge ahead for us," said Nathan Lemphers of the Pembina Institute.

"The unprecedented level of support by British Columbians that they do not want this pipeline and the clear desire from many First Nations in the area that they do not want the pipeline sends a clear signal to Ottawa that if they're going to be approving this pipeline, it's going to be against the wishes of British Columbians."

Tuesday's report raises a list of concerns over the pipeline's safety and its effect on the environment.

The pipeline travels through sensitive ecosystems, including major riverways that are home to the province's iconic salmon, and passes through dangerous mountain terrain that are prone to avalanches and rock slides, the report says.

The project will significantly increase tanker traffic along B.C's northern coast, which the report says will further increase the risk of a spill as massive ships navigate the rugged coastal waters.

The report also claims the chemicals used to dilute bitumen before it is transported make it more corrosive than traditional crude, increasing the risk of corrosion along the pipelines and, consequently, making a spill more likely.

That last claim is the source of considerable debate between the industry and opponents of the oil sands.

A recent report from Alberta Innovates, a research arm of the Alberta government, found no evidence that diluted bitumen is more corrosive than conventional crude, but also recommended more research.

Enbridge spokesman Paul Stanway said many of the report's claims -- including the suggestion that diluted bitumen is somehow more dangerous -- aren't backed up by existing research.

He said groups such as those behind Tuesday's report would be opposed regardless of the measures in place to mitigate the risks.

"If you accept that, then we would not be moving energy around North American at all," said Stanway.

"It depends on where you place the bar. Our aim is to make this a safe project, to build and operate Northern Gateway as safely as humanly possible."

Stanway notes the project is in the midst of a rigorous approval process in an industry that is heavily regulated.

Neither the federal natural resources minister nor B.C.'s energy minister were available for interviews. Written statements from their offices steered clear of the controversy surrounding the pipeline, preferring instead to tout the benefits of the project.

"The proposed Enbridge Project has the potential to provide jobs and stimulate economic activity," the B.C. ministry statement said, adding that the project is currently under review at the National Energy Board.

"We respect this process and believe it offers a meaningful opportunity for stakeholders, concerned citizens and environmental groups to voice their opinions about Enbridge's proposal."

A statement from federal Natural Resources Minister Joe Oliver's office said his department was aware of Tuesday's report, but stressed the importance of the pipeline.

"Minister Oliver has spoken on many occasions about our government's interest in expanding our energy export markets," said the statement.

A federal review panel is preparing to hold public hearings into the 1,200-kilometre pipeline, which, if built, would carry more than a half-million barrels of bitumen -- the heavy crude oil extracted from Alberta's oil sands -- each day to the B.C. coast.

At the same time, the Keystone XL pipeline is facing hurdles of its own. The U.S. State Department has ordered a review of the project, prompting TransCanada to agree to re-route the pipeline away from the environmentally sensitive Sandhills region of Nebraska.