The pipeline is currently in limbo as the State Department weighs whether to grant a presidential permit allowing construction to begin. The pipeline has been on indefinite hold since July 2010, when the U.S. Environmental Protection Agency described a draft environmental study of the project as "inadequate" - raising concerns about greenhouse gas emissions and the potential threat to sensitive ecosystems of a spill. The State Department is now weighing whether to conduct a supplemental eco-study providing more detail on Keystone's emergency response plans, the chemical composition of the tar sands bitumen and potential damage to groundwater from pipeline leaks or spills.

"The choice that the United States faces... is whether to increase its capacity to accept such energy from the most secure, most stable and friendliest location it can possibly get that energy, which is Canada, or from other places that are not as secure, stable or friendly to the interests and values of the United States," said Harper, adding that the U.S. will need far more oil based energy than it can produce.

"What Prime Minister Harper failed to acknowledge is that tar sands oil is highly polluting. There are cleaner, safer ways to meet U.S. energy needs than to import this dirty oil from Canada via a dangerous pipeline through America's heartland," said Alex Moore, dirty fuels campaigner at Friends of the Earth U.S.

Warnings that the pipeline could increase emissions and threaten water supplies have riled up people across the political spectrum in the U.S. including those not normally supportive of environmentalists such as ranchers in Nebraska and former oil refinery workers in Texas.