WATFORD CITY, N.D. -

A man who photographed a North Dakota tornado that heavily damaged several trailers that house oil field workers says the twister remained on the ground for nearly a minute.

William Bunkel told The Associated Press late Monday that he was in Watford City with some co-workers when the tornado warning came.

He says they saw the funnel cloud form several miles to the south and watched it touch down and remain on the ground for nearly a minute before retreating into the sky and disappearing.

The McKenzie County Sheriff's Office says emergency crews are having a difficult time reaching the site and roads are closed to non-emergency vehicles.

People answering the phones at local agencies say they still don't know the extent of the damage or injuries.

The tornado severely damaged about 15 trailers housing workers in North Dakota's oil patch on Monday, the National Weather Service said.

It wasn't immediately clear if anyone was injured when the twister struck the camp about 5 miles south of Watford City, which is in the west of the state about 30 miles southeast of the oil boom hub city of Williston. People who answered the phone at the McKenzie County Sheriff's Office and McKenzie County Hospital in Watford City declined to discuss the matter.

Although the tornado threat for the area had passed, McKenzie County emergency management officials were asking people to stay off the roads because emergency vehicles were having a difficult time reaching the scene.

The oil boom has led to a population explosion in western North Dakota, bringing in tens of thousands of people looking for work. Many reside in hastily-assembled trailer parks or man camps, which contain pre-fabricated structures that can resemble military barracks. Some companies rent blocks of hotel rooms for employees to live in, and some workers sleep in their cars or even tents.

Housing developments are constantly popping up in big areas of town that didn't exist on maps a couple of years ago. But they are still not enough to keep pace with demand and oil money has pushed rents to among the highest in the nation: a simple one bedroom apartment in Williston can easily cost $2,000 a month in rent. Even a spot to park a trailer can cost over $800 per month.