Seven Marines Killed as Mortar Explodes During Training Exercise in Nevada

Seven U.S. Marines were killed and eight were wounded when a mortar exploded at an Army depot in Nevada late on Monday.

The 60 millimeter mortar went off just before 10pm at Hawthorne Army Depot as the men from the 2nd Marine Division were preparing to fire it.

The exact cause of the incident remains under investigation but initial reports described it as a tragic accident during a live-fire training session, NBC News reports.

The injured are being treated for injuries including traumas and fractures at two hospitals. Stacy Kendall, a spokeswoman for Renown Regional Medical Center, said three of the Marines are in a serious condition, with five being described as in a "fair condition."

It is thought the explosion was unrelated to the ammunition stored at the depot.

The depot is a 147,000-acre storage facility located close to the California border, about 140 miles southeast of Reno. It also serves as a training facility for Special Operations forces preparing to deploy to the Middle East.

Maj. Gen. Raymond C. Fox, commander of U.S. Marine Corps Forces Africa, said: "We send our prayers and condolences to the families of Marines involved in this tragic incident. We remain focused on ensuring that they are supported through this difficult time.

"We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice."