LAS VEGAS (Reuters) - An explosion at a U.S. Army munitions depot in western Nevada killed seven Marines from Camp Lejeune, North Carolina and injured seven more during a training exercise, U.S. military officials said on Tuesday.

Army and Marine officials said the cause of the Monday night blast was under investigation, but a military official who spoke on condition of anonymity said initial reports from the scene indicated that a 60 mm mortar round exploded in the tube of a mortar firing point.

The official said three Marines were killed instantly.

Victims were airlifted to Renown Regional Medical Center in Reno, Nevada with injuries that included penetration trauma, fractures and vascular injuries, said Stacy Kendall, a spokeswoman for the medical center. The status of the injured was unclear.

The explosion occurred at 10 p.m. PDT (1.00 a.m. EDT Tuesday) during a Marine training exercise at the Hawthorne Army Depot, about 92 miles southeast of Reno, said facility manager Russ Collier.

Hawthorne Army Depot is a 147,000-acre site used for the storage and destruction of demilitarized ammunition. It was first established as a naval staging area for bombs, rockets and ammunition, and was used by the Navy during most of World War Two. The facility was transferred to the Army in 1977.

The names of the dead and injured will be released following the notification of their families.

The accident came a week after a military plane assigned to a Washington state Naval Air Station crashed during a routine training flight, killing all three crew members on board.

(Reporting by Chris Francescani, David Alexander and Cynthia Johnston; Editing by Paul Thomasch, Nick Zieminski, Steve Gorman, Leslie Gevirtz)