At least seven US troops were killed and several more wounded in an accident during a Marine Corps training exercise in Nevada, the military said Tuesday.
"A fatal incident occurred during a training exercise shortly before 10 pm Monday at Hawthorne Army Depot, Nevada, killing seven servicemembers and injuring several others with 2nd Marine Division," the Marine Corps said.
The cause of the incident was under investigation, officers said, without providing details. Earlier, Marine officers said that an explosion had occurred at a live-fire drill.
Initial reports indicated a 60-millimeter mortar bomb exploded in a tube as Marines were about to fire it.
The vast depot in the desert of western Nevada, which spans 147,00 acres (nearly 600 kilometers), includes more than 2,000 bunkers and its harsh terrain is used to train troops preparing to deploy to Afghanistan.
Major General Raymond Fox, the commander of the II Marine Expeditionary Force, expressed sorrow over the casualties.
"We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice."
The 2nd Marine Division is based in Camp Lejeune, North Carolina.
Defense Secretary Chuck Hagel, who was getting regular updates on the incident, expressed condolences and was "deeply saddened" by the mishap, spokesman George Little told reporters.
Senator Harry Reid of Nevada said: "My heart goes out to the families of those who lost their lives. And my sympathies are with their fellow Marines, who are also grieving this loss."
The site of the Nevada depot, about 140 miles (225 kilometers) southeast of Reno, was chosen for its remote location after a fatal accidental explosion at another depot in New Jersey in 1926.
Hawthorne served as the main staging area for ammunition during World War II and once employed about 5,600 people.
The depot is now run by a contractor and is used to renovate ammunition, prepare troops for desert combat and other training.
The accident came a year after seven Marines were killed in a collision of helicopters near the border of California and Arizona, which involved an AH-1W Super Cobra chopper and a UH-14 Huey.
In one of the worst accidents for the Marine Corps, a helicopter crash in 1989 in the mountains of South Korea claimed the lives of 22 Marines.
The Pentagon has tried to reduce the number of training accidents and says it has made some progress over the past decade. Aviation crashes have declined by about 45 percent since 2002.