Critics also say the process of refining the sticky crude in U.S. refineries spews higher levels of toxic pollutants than conventional oil production, including sulfur dioxide, nitrogen oxides, particulates and heavy metals.
In July, the EPA gave the State Department's for the pipeline the worst rating possible, citing "inadequate information."
The EPA said additional analysis is needed on, among other things, potential greenhouse gas emissions associated with the project, air pollutant emissions at the receiving refineries, pipeline safety and impacts on migratory birds.
The State Department is expected to complete a final environmental review no sooner than early 2011, observers say, though some say it should take longer.
"If they want to be living up to their full duties as government officials, they need to be addressing all of the impacts that EPA identified [that] had not been addressed in the original draft," Alex Moore, fuels campaigner for environmental group Friends of the Earth, told SolveClimate News. "If they were to do that, it would take longer."
TransCanada that "pipelines are the safest, most reliable, economical and environmentally favorable way to transport oil and petroleum products" and says Keystone XL would contribute billions to the U.S. economy.