The proposed Keystone XL oil pipeline is unlikely to increase the pace of Canadian oil sands development, a U.S. State Department study said on Friday, raising pressure on President Barack Obama to approve a project environmentalists see as a major climate change problem.

The massive 11-volume environmental impact study released on Friday did not recommend whether President Barack Obama should grant or deny an application by TransCanada Corp to build the $5.4 billion line, which would transport crude from Alberta's oil sands to U.S. refineries.

But a State Department official who briefed reporters ahead of the report's release said that blocking Keystone - or any pipeline - would do little to slow the expansion of Canada's vast oil patch, maintaining the central finding of the State Department's preliminary study issued last year.

The report's publication opened a new and potentially final stage of an approval process that has dragged for more than five years, taking on enormous symbolic political significance, potentially helping define Obama's legacy.

With another three-month review process ahead and no firm deadline for a decision on the 1,179-mile line, the issue threatens to drag into the 2014 congressional elections in November. Obama is under pressure from several vulnerable Democratic senators who favor the pipeline and face re-election at a time when Democrats are scrambling to hang on to control of the U.S. Senate.

The report reaffirmed the idea that Canada's heavy, bituminous oil sands reserves require more energy to produce and process - and therefore result in higher greenhouse gas emissions - than conventional oil fields.

But after extensive economic modeling, it also found that the line itself would not slow or accelerate the development of billions of barrels of reserves that environmentalists say would exacerbate global warming. That finding is largely in line with what oil industry executives have long argued.

"The approval or denial of any single project is unlikely to significantly affect the rate of extraction of the oil and the oil sands, or the refining of heavy crude on the U.S. Gulf Coast," a State official told reporters ahead of the release.

Secretary of State John Kerry will consult with eight government agencies over the next three months about the broader national security, economic and environmental impacts of the project before deciding whether he thinks it should go ahead. There is no deadline, and the report does not seek to address some of the larger strategic questions involved.

"While we have a lot of deeper and broader analysis in this supplemental (report), it does not answer the broader question about how a decision on this potential pipeline fits in with broader national and international efforts to address climate change," the State official said.