(Adds details of letter, signatories)
WASHINGTON, April 10 (Reuters) - Eleven U.S. Democratic senators on Thursday urged President Barack Obama to make a final decision on whether to approve TransCanada Corp's Keystone XL crude oil pipeline from Canada to the U.S. Gulf Coast no later than May 31.
"We need a definitive timeline laid out, a timeline that reduces the comment period for federal agencies, officials and other entities," the senators wrote to Obama. "We cannot miss another construction season."
The group included several lawmakers expected to face tough re-election battles this year, notably Mary Landrieu of Louisiana, who chairs the Senate Energy Committee.
Also signing on were Heidi Heitkamp of North Dakota, Mark Begich of Alaska, Joe Donnelly of Indiana, Kay Hagan of North Carolina, Joe Manchin of West Virginia, Claire McCaskill of Missouri, Mark Pryor of Arkansas, Jon Tester and John Walsh, both of Montana, and Mark Warner of West Virginia.
The proposed 1,179-mile (1,898-km) line, which would transport crude from Alberta to refineries on the Gulf Coast, has been under U.S. government review since 2008.
Opponents say the pipeline would exacerbate climate change by supporting carbon-intensive development of Canada's oil sands crude. Supporters in Congress and the energy industry say Keystone would improve U.S. energy security and create thousands of construction jobs.
The State Department issued an environmental assessment of the project Jan. 31 that ran to more than 2,000 pages, kicking off a 90-day comment and consultation period during which certain federal agencies can weigh in with their own views.
The senators' letter said that once that comment period ends, Obama should require Secretary of State John Kerry to make a final interest determination recommendation within 15 days, and follow with his own decision shortly after.
"This process has been exhaustive in its time, breadth and scope. It has already taken much longer than anyone can reasonably justify," they said.
(Reporting by Ros Krasny; Editing by Lisa Von Ahn and Bernadette Baum)