ForestEthics said production from Canadian oil sands, also known as tar sands, generates up to five times more greenhouse gas emissions than conventional oil. It said this conflicts with President Obama's pledge to tackle global warming.

"Oil from the tar sands is one of the world's dirtiest," the group's executive director, Todd Paglia, said in a letter to Clinton. "For the U.S., continued dependence on tar sands oil would impair plans to reduce our carbon footprint in the short and long term."

Canada is the biggest foreign oil supplier to the United States and Canada's oil sands are the largest crude deposits outside the Middle East.

The group urged Clinton to deny permits for pipelines that would move the oil to U.S. refineries, particularly the Alberta Clipper pipeline. The State Department has a say in pipelines that would cross the U.S. border.

Enbridge Energy Partners LP's 1,000-mile (1,610-kilometre) pipeline would be able to carry 450,000 barrels of tar sands crude oil a day from Alberta, Canada to Superior, Wisconsin. The oil would then be sent to U.S. refineries to be processed into petroleum products such as gasoline and diesel fuel.

U.S. Energy Secretary Steven Chu said this month that technology will help solve the environmental problems connected with Canada's oil sands production. But ForestEthics said technology will not be able to overcome human health hazards associated with oil sands production.