The company takes issue with the EPA report, which said the recent drop in oil prices will increase Keystone's contribution to greenhouse gases and climate change.
Among other things, TransCanada says the EPA's conclusions aren't supported by the State Department's analysis or by actual market prices and production rates since the company first applied for approval in 2008.
The State Department's approval of the project is required because the pipeline crosses the Canada-U.S. border. The EPA said the department should redo its analysis to take into account the drop in oil prices.
TransCanada CEO Russ Girling suggested last week that the EPA's latest objection is a delay tactic, since oilsands will still be developed even at US$50 a barrel and oil will be delivered to U.S. markets by rail if not by pipeline.
Keystone XL has been held up by a number of court challenges on environmental grounds and faces significant political opposition, including from supporters of President Barack Obama.
The Republican-dominated House is expected to vote on a bill that will likely send the project to Obama, who has promised to use his veto power to prevent Congress from pushing through approval of the pipeline.
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