(c) 2013, Bloomberg News.

(c) 2013, Bloomberg News.

Thousands of North Dakota residents were urged to flee possibly toxic fumes from a fire that engulfed a BNSF Railway Co. train carrying crude oil after it collided with another train, causing a series of explosions.

No injuries to the train crews were reported, Berkshire Hathaway Inc.'s BNSF said in a statement. Two to three railcars were burning and residents living within a five-mile radius of Casselton, North Dakota, were urged to evacuate, Sergeant Dean Haaland of the Cass County Sheriff's Office said by phone at 6:30 a.m. North Dakota time Tuesday.

The accident occurred Monday afternoon near Casselton, about 25 miles (40 kilometers) west of Fargo. As of 9:20 p.m., 21 railcars were ablaze, according to a statement from BNSF.

The National Transportation Safety Board sent a team to investigate, the agency said in an emailed statement Tuesday. Richard Hipskind, a rail safety specialist, will serve as the investigator-in-charge. Board member Robert L. Sumwalt is accompanying the team and will serve as the principal spokesman during the on-scene phase of the investigation, the NTSB said.

"There is still a small blaze out there," Haaland said at about 4 a.m. "We are waiting for sunrise to evaluate whether they will go in and extinguish the fire, or if it will burn itself out."

The accident is the fourth major North American derailment in six months by trains transporting crude. Record volumes of oil are moving by rail as shale from North Dakota to Texas have pushed U.S. output to the most since 1988 and pipeline capacity has failed to keep up.

BNSF, based in Fort Worth, Texas, didn't provide an estimate for when the tracks will reopen. Berkshire's Burlington Northern Santa Fe railroad carried about 500,000 barrels of oil a day in March, Chief Executive Officer Warren Buffett said at the time.

A westbound train carrying soybeans derailed west of Casselton just after 2 p.m., said Cecily Fong, a spokeswoman for the state Emergency Services Department. An eastbound train carrying oil hit the derailed train, causing the fire, she said.

The train carrying crude included 106 cars, BNSF said. About half of the cars detached from the wreck and the rest were involved in the accident, Fong said.

One railcar can hold about 700 barrels of oil, according to the Energy Information Administration, the Energy Department's statistical arm.

The Federal Aviation Administration banned flights below 6,000 feet above sea level in the area of the incident, citing possible tank car explosions.

In November, a train carrying oil to the Gulf Coast from North Dakota derailed in Alabama, touching off fires. A month earlier, residents were evacuated from a rural area of Alberta after 13 railcars, four of which were carrying crude, derailed and ignited a blaze.

In July, a runaway train transporting crude exploded and killed 47 people in the Quebec town of Lac-Megantic.

The derailment "is likely to flare up the debate on the environmental side of the shale oil boom, which could result in higher costs for the industry," JBC Energy, a Vienna-based energy consultant, said in an emailed report Tuesday.

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