Six advocacy groups have asked the State Department to prepare a new environmental review of the proposed Keystone XL oil pipeline, saying that evidence has emerged showing it will hurt the environment.

The demand, contained in a 48-page letter, comes as President Obama has pledged to block the project, which would carry heavy crude from Canada to the Gulf Coast, if it would “significantly exacerbate the climate problem.”

The letter sent Monday says that several new analyses show that the project will speed heavy crude extraction in Canada’s oil sands region. Activists say the State Department should refuse to approve the pipeline because of adverse impact on the environment.

The State Department is currently responding to more than 1.2 million comments on a draft environmental assessment issued in March, which suggested that denying a permit to the pipeline firm Trans­Canada would have little overall climate impact because the oil would be extracted and shipped out anyway.

“Limitations on pipeline transport would force more crude oil to be transported via other modes of transportation, such as rail which would probably (but not certainly) be more expensive,” the assessment said.

The six environmental advocacy groups — Bold Nebraska, the Center for Biological Diversity, the National Wildlife Federation, the Natural Resources Defense Council, Oil Change International and the Sierra Club — said that conclusion relies on “an overly-simplistic, outdated view of a rapidly-changing oil market.”

They cited several reasons for redoing the assessment’s climate analysis, including a Goldman Sachs report that questions the extent to which rail shipments could replace the pipeline in the near term and a Royal Bank of Canada estimate that oil sands development would be jeopardized by denying the project.

The State Department declined to comment on the matter Thursday. TransCanada and other pipeline proponents dismissed the environmentalists’ analysis as flawed.

The advocacy groups said that the Environmental Protection Agency has suggested that the State Department played down the amount of greenhouse gas emissions linked to the project’s construction. The groups called on State Department officials to take into account the higher “social cost of carbon” the administration is now using, which is intended to reflect the negative climate impact of activities that release carbon into the atmosphere.

In an e-mail, TransCanada spokesman Shawn Howard called the criticism “just one more example of the professional activists looking for reasons to delay the project when the facts do not support their claims.”

He noted that Canadian crude producers have ordered rail cars capable of moving more than 10 million barrels of oil and that six companies are in the process of building or expanding 16 rail terminals in western Canada.

Oil Sands Fact Check, a group funded largely by oil companies and manufacturing, chemical and other firms, issued a statement noting that the Goldman Sachs report cited by environmentalists said that analysts “believe risk would grow that Canadian heavy oil/oil sands supply would remain trapped in the province of Alberta” if Keystone XL was stopped, not that it would be stranded indefinitely.