BILLINGS, Mont. — U.S. safety officials say the dangers posed by a sharp spike in oil shipments by rail in North America extend beyond shipments from the booming Bakken region of North Dakota and Montana, and include oil from elsewhere in the U.S. and Canada.

Acting National Transportation Safety Board Chairman Chris Hart says all crude shipments are flammable and can damage the environment – not just the Bakken shipments involved in a series of fiery accidents.

He cited recent accidents in Mississippi, Minnesota, New Brunswick and Pennsylvania that involved oil shipments from Canada, and said they exemplify “the risks to communities and for the environment for accidents involving non-Bakken crude oil.”

Hart’s comments were contained in a letter to U.S. Sens. Ron Wyden and Jeff Merkley obtained by The Associated Press. They add to growing pressure on federal regulators to improve oil train safety in the wake of repeated derailments, including in Lac-Magentic, Quebec, where 47 people were killed in a massive conflagration.

Transportation Secretary Anthony Foxx last month ordered railroads to notify states of Bakken oil shipments so firefighters and first responders can better prepare for future accidents.

But Wyden and Merkley told Foxx Thursday that his order leaves emergency personnel in the dark on oil shipments from outside the Bakken or smaller than the 1,000,000 gallon threshold set by Foxx.

The Oregon Democrats urged an expansion of the order to cover crude from all parts of the U.S. and Canada, and to lower the threshold to include smaller shipments.

“With the exception of the Lac-Megantic accident, every accident involving crude oil, ethanol and other flammable materials since 2006 has resulted in a hazardous materials release of less than 1,000,000 gallons,” Wyden and Merkley wrote in a Thursday letter to Foxx. They said the examples cited by the NTSB chairman show trains that carry non-Bakken crude and less than 1,000,000 pose the same “imminent hazard” that Foxx has asserted for Bakken oil.

Representatives of the oil industry and officials in North Dakota also complained about Bakken oil being singled out by regulators – although for opposite reasons. The American Petroleum Institute and American Fuel and Petrochemical Manufacturers have said Bakken oil is no more volatile than other light, sweet crudes produced elsewhere in the U.S. and abroad.

About 700,000 barrels of oil a day – enough to fill 10 trains of 100 cars each – is coming out of the Bakken by rail, according to the North Dakota Pipeline Authority. That equals roughly 80 percent of crude-by-rail shipments nationwide during the first three months of the year.

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