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The lengthy document also draws no conclusions on whether the pipeline is in the United States's economic and energy interests, a determination to be made later this year by President Obama. But it will certainly add a new element to the already robust climate change and energy debate around the $7 billion proposed project.

The study will help guide the president's decision, but it does not make the politics any easier. Environmental advocates and landowners along the route have mounted noisy protests against the project, including a large demonstration in Washington last month, and view Keystone as a test of Mr. Obama's seriousness about addressing global warming.

The president faces equally strong pressure from industry, the Canadian government, many Republicans and some Democrats in Congress, local officials and union leaders, who say the project will create thousands of jobs and provide a secure source of oil that will replace crude from Venezuela, Saudi Arabia and other potentially hostile suppliers.

The draft report, which updates a 2011 study that essentially gave the project a green light, carefully weighs the impacts of the pipeline, which would carry about 800,000 barrels a day of heavy crude oil from tar sands formations in Alberta across the Great Plains to Gulf Coast refineries.

President Obama rejected the original route proposed by the pipeline operator, TransCanada, because of potential adverse impacts on sensitive grasslands and aquifers in Nebraska. The new environmental statement looks at a revised route submitted by TransCanada last spring.

The new impact statement says that extracting, shipping, refining and burning oil from the tar sands produces more climate-altering greenhouse gases than most conventional oil, but less than many of the project's critics claim. The State Department study says that tar sands oil produces 5 percent to 19 percent more greenhouse gas emissions than other crude, depending on what oil was compared and who performed the calculations.

It predicts that Canada and its oil industry partners will probably continue to develop the oil sands even if the Keystone XL pipeline is not built. It states that building or not building the pipeline will have no significant impact on demand for heavy crude in the United States.

And it says that alternate means of transporting the oil – rail, truck, barge – also have significant environmental and economic impacts, including higher cost, noise, traffic, air pollution and the possibility of spills. It does not say that one method is better for the environment than the other, but says that a spill is more likely for rail transport but the volume of oil spilled from a pipeline probably would be greater.

Publication of the document next week starts a 45-day public comment period and then a protracted review before a final impact statement is issued, meaning a presidential decision on the project is still months away.

Jane Kleeb, director of Bold Nebraska, an advocacy organization that is adamantly opposed to the pipeline, said her group had wanted a 120-day period for Nebraskans to be able to comment on the supplemental environmental report. She said 45 days was not enough time, especially because local cattle ranchers were in the middle of calving season.

Ms. Kleeb said that federal reviews of the pipeline have not yet addressed longstanding concerns about spills and the overall impact of Keystone XL on the environment.

"All of the risks remain for Nebraskans," she said. "There is no scientific study as to what a tar sands spill will do to our water and our soil. And our landowner property rights continue to be ignored by both state and federal officials."

State environmental officials in Nebraska conducted a review of the revised route and said that the risks to the state's land and waters were manageable. Based on that report, Gov. Dave Heineman, a Republican, has voiced support for the pipeline and urged the Obama administration to approve it.