WASHINGTON, Dec. 30 -- A train carrying crude oil collided with another train in the northern U.S. state of North Dakota, sparking a large fire, local media reported Monday.

So far there were no reports of casualties, but local authorities urged residents nearby to take shelter due to huge flames and heavy smoke, U.S. broadcaster CNN reported.

Firefighters would be forced to let the fire burn out, because they could not get close enough to the flames, according to CNN reports.

It was unclear how the accident happened, and authorities were trying to determine whether one of the trains had derailed before they collided.