The Pentagon has banned the use of 60mm mortar rounds by its troops worldwide after seven US marines were killed by a misfire at a training ground in the Nevada desert.

The explosion at Hawthorne army depot on Monday killed seven marines. There were reports that an eighth died later in hospital but this was not immediately confirmed by the military. Eight people survived with injuries.

The mortar round exploded in its firing tube during an exercise, said Brigadier General Jim Lukeman at a news conference in North Carolina, where the marines are based. Investigators were trying to determine why it went off.

The Hawthorne depot is used to train troops heading overseas. The rescue of the wounded marines was complicated by the remoteness of the site, which is favoured because its harsh geography simulates conditions in Afghanistan.

"We send our prayers and condolences to the families of Marines involved in this tragic incident," said the 2nd Marine Expeditionary Force commander, Major General Raymond C Fox. "We mourn their loss and it is with heavy hearts we remember their courage and sacrifice."

The 60mm mortar traditionally requires three to four marines to operate but it is common during training for others to observe nearby. The firing tube is a shell some 355mm (14 ins) in length.

The mortar has changed little since the second world war and remains one of the simplest weapons to operate. Joseph Trevithick, a mortar expert with Global Security.org, said its simplicity meant it was depolyed at the lowest level of infantry units.

A number of things could go wrong including a fuse malfunctioning, a problem with the barrel's assembly or a round prematurely detonating inside the tube, Trevithick said.

A 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 was not unusual and would last until the investigation determined it was safe to start using the weapon again.