Canadian Prime Minister Stephen Harper has written to President Obama to say that he is prepared to work on a joint plan between the two countries to reduce carbon emissions, in an attempt to secure approval of the Keystone XL pipeline project, the Canadian Broadcasting Corp. reported Friday.

Harper reportedly wrote to Obama in late August, the Wall Street Journal reports, signaling his willingness to accept carbon-reduction targets proposed by the U.S. and to address concerns raised by the White House about Keystone and its impact on the environment.

The proposed Keystone XL project would carry heavy crude from Alberta to the U.S. Gulf Coast, and has triggered intense opposition from environmentalist groups concerned that it will contribute to climate change. The project is presently under review by the U.S. State Department, and will require final approval from President Obama.

Harper’s letter immediately triggered a strong reaction, with Thomas Pyle, President of the Institute for Energy Research in Washington D.C., slamming the letter in a statement as a “capitulation to the White House political machine that has unnecessarily delayed the project for almost five years.”

Pyle criticized the U.S. government too, saying: "At the end of the day, White House officials are more concerned about cutting a deal with environmental extremists than they are with the Canadian prime minister.”

Meanwhile TransCanada Corp., which proposed the controversial Keystone XL project, released a more hopeful statement:

“Despite three years of misinformation, an overwhelming majority of Americans continue to support Keystone XL because they understand that this is a choice about where to get oil from: a secure, stable and reliable ally in Canada, or continue to import higher-priced, ‘conflict oil’ from the Middle East and Venezuela – where American values and interests are not shared or respected.”

A spokesman for Harper declined to comment on the letter, according to the Wall Street Journal. Reuters reported that the White House has not yet responded to Harper’s letter.

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Reuters contributed to this report.