(CNN) -- An explosion occurred Monday in a West Virginia mine, leaving more than 20 people unaccounted for, said Jimmy Gianato, director of homeland security for the state.

West Virginia's state mine director reported that six coal miners were dead and 21 were missing after the explosion in Raleigh County, according to CNN affiliate WCHS.

The explosion happened at about 4:30 p.m. at Massey Energy's Performance Coal Co. mine in Whitesville, 30 miles south of Charleston, it said.

WCHS: Six deaths reported

The company identified the facility as Upper Big Branch Mine.

Rescue teams and state and federal agencies were responding.

Massey Energy Co., based in Richmond, Virginia, has operations in West Virginia, Kentucky and Virginia. It is the largest coal producer in Central Appalachia, it said in a statement.

Blast reported at West Virginia mine; More than 20 unaccounted for - CNN.com