At least seven Marines died after a mortar round accidentally exploded during training in Nevada, according to a Marines spokesman.

The Marine Corps said several other Marines were injured in the incident, which occurred Monday evening at the Hawthorne Army Depot, about 100 miles southeast of Reno. The injured Marines were taken by helicopter to Renown Regional Medical Center in Reno, the hospital said.

Marine Corps officials said the incident occurred when a mortar round exploded in the mortar tube during a mountain training exercise.

Lt. Col. Neil Murphy, a Marines spokesman, said that authorities are treating the explosion as an accident and are investigating the cause. Authorities gave no information as to whether the explosion might have occurred because of equipmentfailure or because of human error.

Hundreds of U.S. troops across all services die each year in accidents, although the majority of those aren't training-related. Before Monday, the most recent fatal training exercise involving Marines happened last April when two Osprey aircraft crew members died in a crash in Morocco, according to a Corps spokesman. In February of last year, seven Marines died when two helicopters collided near Yuma, Ariz.

This week's incident involved a 60mm mortar