An eighth US Marine has died after a training exercise ended with an exploding mortar shell causing the Pentagon to immediately halt the use of the weapon worldwide until an investigation can determine its safety.

It happened at Nevada's Hawthorne Army Depot, a facility used by troops heading overseas, during an exercise involving the 2nd Marine Expeditionary Force at Camp Lejeune.

Several Marines from the unit were injured in the blast, authorities said.

It was not immediately clear whether the 60mm mortar shell exploded prematurely inside its firing tube or whether more than a single round exploded, the official said.

Eight men under the age of 30 were taken to Renown Regional Medical Center in Reno.

One of them died, four were in serious condition, two were in fair condition and another was discharged, said spokesman Mark Earnest.

John Stroud, national junior vice commander in chief for the Veterans of Foreign Wars, began a memorial event in Hawthorne later by saying "one of the critical has passed," bringing the death toll to eight.

Mourners then laid eight floral arrangements at a park where a flag flew at half-staff within sight of the Hawthorne depot's boundary.

Stroud said he spoke with Marine officers from Camp Lejune who gave him the news before the ceremony.

The identities of those killed won't be released until 24 hours after their families are notified.

"We send our prayers and condolences to the families of Marines involved in this tragic incident," said the force's commander, Major General Raymond Fox.

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

The rescue was complicated by the remoteness of the site, which is favoured because the harsh geography simulates conditions in Afghanistan.

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.

The firing tube is supported in a tripod-like design and fires roughly a 1.3kg shell, about 35cm in length and a bit larger than 5cm in diameter.

The mortar has changed little since World War II and remains one of the simplest weapons to operate, which is why it is found at the lowest level of infantry units, said Joseph Trevithick, a mortar expert with Global Security.org.

"Basically, it's still a pipe and it's got a firing pin at the bottom," Trevithick said.

Still, a number of things could go wrong from a fuse malfunctioning, a problem with the barrel's assembly to a round prematurely detonating inside the tube, he said.

The Marine Corps official said an explosion at the point of firing in a training exercise could kill or maim anyone inside or nearby the protective mortar pit and could concussively detonate any mortars stored nearby in a phenomenon known as "sympathetic detonation."

The official said a worldwide moratorium after such an accident is not unusual and would persist until the investigation determines that the weapon did not malfunction in ways that would hurt other Marines or that mortars manufactured at the same time as the one involved in the accident were safe.

The official said it would be normal to warn other US military branches that use 60mm mortars, such as the Army, about the Marines warning.

The moratorium could last for weeks or months.

The investigation will focus on whether the Marines followed procedures to properly fire the weapon, whether there was a malfunction in the firing device or in the explosive mortar itself, the official said.

The Hawthorne Army Depot stores and disposes of ammunition.

The facility is made up of hundreds of buildings spread over more than 595 square km, and bunkers dot the sagebrush-covered hills visible from the highway. Hawthorne is in the shadow of Mt Baker, which reaches an elevation of 11,239 feet.

Retired Nevada state archivist Guy Rocha said the facility opened in 1930, four years after a lightning-sparked explosion virtually destroyed the Lake Denmark Naval Ammunition depot in northern New Jersey.

The blast and fires that raged for days heavily damaged the adjacent Picatinny Army Arsenal and surrounding communities, killing 21 people, and seriously injuring more than 50 others.

- AP