“No, I don’t think that any one issue is a disaster for the climate, nor do I think there is one solution for the climate change challenge that we have,” McCarthy said during an interview with POLITICO’s Mike Allen.

Keystone critics have long alleged that the pipeline, if approved, would greatly exacerbate climate change.

And Environmental Protection Agency’s concerns about Keystone’s climate impact have given ammunition to environmentalists fighting the project. In comments to the State Department released in February, the agency said state should give “additional weight” to whether the sharp drop in oil prices in recent months would increase the pipeline’s environmental impact and stimulate production in the carbon-rich Canadian oil sands.

McCarthy, in her interview with POLITICO, stressed that those EPA comments did not come to any conclusion about the pipeline, and she pushed back on the criticism from Canada’s ambassador to the U.S., Gary Doer.

“I have great respect for the ambassador, but he should just relook at the comment letter that we put in,” she said.

TransCanada spokesman Mark Cooper countered that despite the EPA’s comments, the current downturn in oil prices would “not significantly impact whether the oil sands will be developed.” In addition, Cooper noted, Keystone’s 700,000-plus barrels of heavy crude imports would displace “foreign heavy oil that produces similar or greater amounts of” emissions.

One anti-Keystone group said McCarthy’s statements appeared to diverge from what the agency told the State Department, which could finish its long-delayed review of Keystone within weeks or even days.

“Gina McCarthy would do well to look at comments published by her own EPA, warning that Keystone XL would accelerate development of the tar sands oil field in Canada, which in turn would mean game over for our climate,” Karthik Ganapathy, spokesman for the green group 350.org, said by email.

Once Secretary of State John Kerry finishes weighing whether Keystone’s construction is in the national interest, President Barack Obama is poised to make the final call on the pipeline — a decision that has no binding deadline.

Other green groups said they didn’t see McCarthy’s Keystone comments Monday as a shift in the EPA’s view. Jim Murphy of the National Wildlife Federation said he didn’t find her statement “concerning,” adding that the EPA “has been strong throughout” in pointing to the potential greenhouse gas emissions generated by the $8 billion pipeline.

Yet McCarthy’s prominent role in Obama’s strategy to make climate change a central part of his legacy adds extra weight to her words.

Don Stewart, spokesman for Senate Majority Leader Mitch McConnell, circulated McCarthy’s comments on Twitter, adding that the EPA chief appeared to have “debunked the anti-#KeystoneXL crowd’s alarmism today. Cool.”