Image caption Brig Gen Jim Lukeman said the mortar round exploded in the firing tube

Seven US Marines have been killed and eight injured after a mortar shell exploded unexpectedly in a training accident in the US state of Nevada.

The blast occurred on Monday night at the Hawthorne Army Depot, a Marine Corps official said.

At least eight men are in hospital, three in a serious condition and five in a stable condition.

The shell discharged in its tube but it is not clear if more than one round exploded, a Marine spokesman said.

At a news conference, Brig Gen Jim Lukeman said the cause of the malfunction was under investigation.

The US Marines Corps has banned use of all 60mm mortars in both training and combat until the review establishes what went wrong. The weapon normally needs three or four troops to operate.

Those caught in the blast at 22:00 local time (05:00 GMT on Tuesday) were members of the Second Marine Expeditionary Force.

Renown Regional Medical Center in Reno said it had received nine patients, one of whom died.

All the patients were men under the age of 30, who suffered penetrating trauma, fracture and vascular injuries, said Stacy Kendall, a spokeswoman for the facility.

Second Marine Expeditionary Force commander Maj Gen Raymond Fox said in a news release: "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."