An international alliance of environmental groups has joined the growing chorus of opponents to a massive oil pipeline proposal for northern B.C.

A collection of groups including the Natural Resources Defense Council and the Pembina Institute released a report Tuesday arguing that the $5.5-billion Enbridge Northern Gateway pipeline poses an unacceptable risk to the province's mountain rivers and pristine coastline.

Catherine Stewart of the Living Oceans Society says the pipeline would threaten the Great Bear Rainforest -- the only place in the world where the white spirit bear can be found.

"It's just a huge gamble for very little return," she told CTV News.

"There are a host of concerns. The pipeline would be running through extremely treacherous terrain prone to landslides."

The pipeline would pump half a million barrels of bitumen from the oil sands in Alberta through to Kitimat, where it would be loaded onto supertankers bound for Asia.

First Nations groups and environmental activists have been speaking out against the project for years. They were emboldened this month when the American Keystone XL pipeline was sidelined because of potential impacts on the environmentally sensitive Sandhills region of Nebraska.

B.C. First Nations opposed to the Northern Gateway project are holding a news conference Thursday to discuss the pipeline.

"Oil can never be cleaned up. I've seen it on the coastlines -- gobs of oil coming up to the coastline, wildlife being destroyed," said Geraldine Thomas-Flurer of the Yinka Dene Alliance.

Enbridge disputes the environmental claims and says it's working on revenue-sharing deals with many First Nations.

A decision on the project is likely more than a year a year away, but it's quietly being endorsed by governments in Ottawa and Victoria, where officials are tempted by jobs and economic spinoffs.

But BC NDP environment critic Rob Fleming says the benefits aren't worth the risk.

"There are some environmental challenges that cannot be mitigated. There are some risks that are too great," he said.

With files from The Canadian Press and a report from CTV British Columbia's Jim Beatty