Seven Marines from a North Carolina unit were killed and several injured after a 60mm mortar exploded unexpectedly during a training accident at the Hawthorne Army Depot in western Nevada, a Marine Corps official said Tuesday.

The Marines immediately issued an indefinite moratorium on firing of all such mortars worldwide until an investigation clears as safe the type of weapon and ammunition in the tragedy.

The official, speaking on condition of anonymity, said it was not immediately clear whether the mortar exploded prematurely inside its firing tube or whether more than a single round exploded. The official was not authorized to speak to a reporter about the accident.

The 60mm mortar is a weapon that traditionally requires three to four Marines to operate, but it's common during training for others to observe nearby.

Marines remembered 'with heavy hearts'

The cause of the accident shortly before 10 p.m. local time Monday is under investigation, officials said in a statement from the 2nd Marine Expeditionary Force at Camp LeJeune, N.C.

In this handout photo from the U.S. Marines, mortarmen with the 23rd Marine Regiment fire off illumination rounds as part of their training at the Hawthorne Army Ammunition Depot. Seven Marines from a North Carolina unit were killed and several injured Tuesday after a mortar exploded unexpectedly during a training accident at the western Nevada facility. (U.S. Marine Corps/Lance Cpl. M. C. Nerl/Reuters)

The injured were taken to area hospitals for treatment and further evaluation. A spokesman for Renown Regional Medical Center in Reno, the area's major trauma hospital, couldn't immediately confirm how many people were being treated there. A nursing supervisor at Saint Mary's Regional Medical Center said her hospital hadn't taken patients from the incident

"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," said the force's commander, Maj.-Gen. Raymond C. Fox. "We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice."

The identities of those killed were not released pending notification of their families and a 24-hour waiting period after that.

Special forces training facility

The Hawthorne Army Depot stores and disposes of ammunition. The facility is made up of hundreds of buildings spread over more than 596 square kilometres.

Hawthorne has held an important place in American military history since World War II when it became the staging area for ammunition, bombs and rockets for the war. The Nevada Division of Environmental Protection says that the depot employed more than 5,500 people at its peak. Nevada was chosen for the location because of its remoteness in the wake of a devastating explosion at the government's main depot in New Jersey in the 1920s.

It opened in September 1930 as the Naval Ammunition Depot Hawthorne, was redesignated Hawthorne Army Ammunition Plant in 1977 when it moved under the control of the Army, according to its website. In 1994, the site ended its production mission and became Hawthorne Army Depot. The site currently serves several purposes for the military, including storing ammunition and explosives and providing what the military calls an ideal training facility for special forces preparing for deployments to similar desert terrain in places like Afghanistan.

Nevada Republican Senator Dean Heller tweeted: "Thoughts and prayers are with the families who lost a loved one in the Hawthorne Army Depot explosion. Grateful for their service."