WASHINGTON — A government warning about the dangers of increased use of trains to transport crude oil is giving a boost to supporters of the long-delayed Keystone XL pipeline.

U.S. and Canadian accident investigators urged their governments Thursday to impose new safety rules, warning that a “major loss of life” could result from an accident involving the increased use of trains to transport large amounts of crude oil.

Pipeline supporters said the unusual joint warning by the U.S. National Transportation Safety Board and the Transportation Safety Board of Canada highlights the need for Keystone XL, which would carry oil derived from tar sands in western Canada to refineries on the U.S. Gulf Coast. Oil started flowing Wednesday through a southern leg of the pipeline from Oklahoma to the Houston region.

Sen. John Hoeven, R-N.D., said the yearslong review of Keystone has forced oil companies to look for alternatives to transport oil from the booming Bakken region of North Dakota and Montana to refineries in the United States and Canada. A planned spur connecting Keystone to the Bakken region would carry as much as 100,000 barrels of oil a day.

“Clearly because this project has been held up, that is creating more (oil) traffic by rail,” Hoeven said Thursday. “Those companies are being forced to deliver their product by rail because they don’t have the pipelines.”

A pipeline opponent said Hoeven’s argument is based on a false choice between moving oil by rail or pipeline.

“It’s disingenuous for supporters of Keystone XL to suggest that if we build Keystone, we won’t have safety risks posed by crude-by-rail, and if we don’t build the pipeline we will” have those risks, said Anthony Swift, an attorney for the Natural Resources Defense Council who has studied the Canadian tar sands.

Shipment of oil by train is likely to continue, whether or not Keystone XL is approved, Swift and others said, as companies seek to capitalize on an oil boom that has pushed North Dakota to become the second-largest oil producing state after Texas.

Both rail and pipelines have good overall safety records, although several high-profile accidents involving crude oil shipments – including a fiery explosion in North Dakota last month and an explosion that killed 47 people in Canada last year – have raised alarms.

Spills from rail cars occur more frequently than from pipelines but tend to be smaller.

Pipelines also can be built to avoid large population centers and fragile ecosystems, while crude oil-carrying trains frequently travel through large cities such as Detroit and Philadelphia.

Chicago Mayor Rahm Emanuel on Thursday called for new steps to protect communities from accidents involving oil trains and other hazardous materials, including fees on companies that ship crude oil by rail and on industries that use oil.

The money would go into a fund to rebuild rail lines, Emanuel told a meeting of the U.S. Conference of Mayors in Washington. Chicago is a major freight rail hub.

Emanuel’s proposal was endorsed by the mayors of Philadelphia, Madison and Milwaukee, Wis., Kansas City, Kan., and Peoria, Ill.