Scientists from some of the most prominent institutions in the United States have written a letter to President Barack Obama urging him not to approve a pipeline that would deliver bitumen from Alberta's oilsands.

"We are writing to add our voices to the indigenous leaders, religious leaders and environmentalists calling on you to block the proposed Keystone XL pipeline from Canada's tarsands," says the letter, released Wednesday.

It's signed by 19 researchers from universities such as Princeton, Harvard, Cornell, Stanford and Columbia, as well as facilities such as the Woods Hole Research Center and the Scripps Institute of Oceanography. Signatories include geophysicists, oceanographers, climatologists, biologists and engineers.

The U.S. State Department is reviewing the $7-billion US pipeline that would be built by TransCanada PipeLines Ltd. A preliminary environmental assessment is expected by mid-August.

The pipeline would transport Alberta oilsands crude through the American heartland to refineries on the Gulf Coast. The State Department's final decision is expected by the end of the year.

Secure souce

Advocates say the line would give the U.S. a secure energy supply and provide Canada with an assured market.

But others say the pipeline would be unsafe and would lock the U.S. into a high-carbon source of oil at a time when the country is trying to reduce greenhouse gas emissions.

"When other huge oilfields or coal mines were opened in the past, we knew much less about the damage that the carbon they contained would do to the Earth's climate system and to its oceans," the letter says. "Now that we do know, it's imperative that we move quickly to alternate forms of energy — and that we leave the tarsands in the ground."

Last week, lawmakers in the Republican-controlled House of Representatives passed a bill to force Obama to make a decision on the project by Nov. 1. That bill now faces the Democrat-controlled Senate, where many members are vehemently opposed to the pipeline.

Daniel Kessler of the Rainforest Action Network, which distributed the letter, said it was organized by American author and environmentalist Bill McKibben.

TransCanada isn't involved in the production of the diluted bitumen that would fill the line, company spokesman Terry Cunha pointed out.

"The key focus of Keystone is the safe, secure delivery of energy to the U.S.," he said.

He said the United States imports about two-thirds of its energy needs and has to decide which country it wants to buy from.