SMACKOVER, Arkansas (CNN) -- An explosion at an oil refinery near Smackover, about 100 miles south of Little Rock, killed three people Wednesday, police said.

Capt. Larry Gleghorn of the Arkansas State Police said the explosion happened around 10:30 a.m. CST (11:30 a.m. EST) as a cleaning crew from an independent contractor was working on a valve on a naphtha tank at the Cross Oil Refinery.

Three members of that crew were killed.

Three other people suffered burn injuries, according to Stephanie Burton of the Southwest Arkansas Medical Center in nearby El Dorado.

The blaze was put out by the refinery's fire department.

Naphtha is a flammable, volatile, oily liquid that is used as an industrial solvent.

A spokeswoman for the Arkansas Department of Pollution Control and Ecology told CNN that the environment was not threatened by the explosion.

Spokeswoman Rhonda Sharp said one of her agency's inspectors reported that all the runoff from foam used to extinguish the blaze and water to cool down other tanks had been contained and the surrounding environment was not harmed.