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WASHINGTON -- The head of the U.S. Environmental Protection Agency resigned abruptly last week, reportedly to protest the Obama administration's apparent plans to approve TransCanada's Keystone XL pipeline in the coming months.

The American environmental movement is abuzz following a New York Post report that Lisa Jackson suddenly quit the post because she doesn't want to be at the helm of the agency when the White House rubber-stamps the controversial project.

That could happen as early as March or April, the paper suggested.

"She was going to stay on until November or December," a source close to Jackson told the Post.

"But this changed it. She will not be the EPA head when Obama supports (Keystone XL) getting built."

The EPA is one of several federal agencies that's been advising the Obama administration on the $7-billion pipeline, a project that would carry millions of barrels of crude a week from Alberta's carbon-intensive oilsands to the U.S. Gulf Coast.

The ultimate decision on Keystone XL's fate rests with the State Department, since the pipeline crosses an international border.

Jackson is Obama's top environmental adviser, appointed head of the agency soon after his inauguration in 2008. Over the past four years, she's pushed through the toughest new air and water pollution rules in more than 20 years and has frequently spoken out on climate change.

Under her watch, the EPA has also raised serious concerns about Keystone XL.

In July 2010, as TransCanada awaited a decision from the White House on the pipeline, the EPA sent a letter to the State Department calling its draft environmental assessment of the project "inadequate."

It chastised analysts for failing to address the greenhouse gas emissions associated with Keystone XL. The letter also urged the State Department to further examine pipeline safety and spill-response planning, as well as the impact on Canadian native communities.

In October 2011, Jackson reiterated those concerns in an appearance at Howard University in Washington.

"This isn't a little tiny pipeline, this is a pipeline that cuts our country literally in half," she said.

Obama rejected the pipeline early last year, but invited TransCanada to file a new application with an altered route that would skirt Nebraska's ecologically sensitive Sand Hills region.

TransCanada did so, and is now awaiting word on approval from the State Department after getting the all-clear from the state of Nebraska.

Pipeline advocates say the project will create much-needed jobs in the U.S. Midwest, and help end American dependence on oil from hostile OPEC regimes.

In the statement announcing her resignation, the EPA said Jackson wanted to "pursue new challenges, time with her family and new opportunities."

A Capitol Hill source doubts Jackson's resignation is Keystone-related, suggesting instead she's stepping down due to job fatigue.

Jackson has been a "javelin-catcher on a million issues," the source said.

Being head of the environmental watchdog is a thankless job, because "Congress hates the EPA," the source added.

"There's a joke that she has her own parking space on Capitol Hill because she's here so often getting beaten up on so many issues."

The real threat to Keystone, the source added, could be John Kerry, expected to soon be easily confirmed America's new secretary of state following Hillary Clinton's resignation.

Kerry is one of the most vocal climate change hawks in Congress, and has long stressed the need for the U.S. to combat its addiction to fossil fuels. His nomination by Obama has delighted American environmentalists.

In October 2011, Kerry, the head of the U.S. Senate's foreign relations committee, vowed that no stone would remain unturned as senators examined the environmental impact of Keystone XL.

"There's a lot at stake here and I'll do my best to leave no question unanswered including every possible economic and environmental consideration before a final decision is made," he said in a statement.

Keystone XL places the longtime Massachusetts senator in a quandary.

"He doesn't want to start off as the top diplomat of the country angering Canada," the source said. "But there's no question that a Kerry State Department is not going to be helpful in terms of Keystone."

Shawn Howard, a spokesman for TransCanada, said the company is prepared to arm both Kerry and Jackson's eventual replacement with as much information as possible, pointing specifically to Nebraska's recent thumb's up to Keystone XL's new route.

"We will work with Secretary Clinton's successor, as well as Ms. Jackson's, to provide them with the information that is required for a decision to be made on our presidential permit application," he said.