"As you recently stated, tar sands oil is 'dirty oil'. Approval of this pipeline will significantly increase our dependence on this oil for decades. We believe the Department of State (DOS) should not pre-judge the outcome of what should be a thorough, transparent analysis of the need for this oil and its impacts on our climate and clean energy goals," the Senators wrote.

"We applaud Senator Leahy's leadership in championing clean energy over more dirty, dangerous oil," said Alex Moore, dirty fuels campaigner for Friends of the Earth.

"This powerful letter is another example of how the largest oilsands customer, the United States, is carefully watching oilsands development in Canada. It is time the Alberta and federal governments strengthened and enforced laws and regulations to reduce water and greenhouse gas pollution, and damage to forests and wildlife from oilsands development," stated Danielle Droitsch, U.S. Policy Director for the Pembina Institute.

The Keystone XL pipeline would be constructed by Canadian oil and gas giant TransCanada. If approved, the pipeline will move oil from Canada through the U.S. plains to Gulf Coast refineries near Houston at a rate of 900,000 barrels per day. The pipeline has been opposed by environmental, agricultural, and tribal organizations. More than 50 members of Congress have also voiced strong concerns, and more than 48,000 activists urged the Obama administration to reject the pipeline during the State Department's public comment period.