Four US marines have been killed in an accident while clearing a training area of obstructions at Camp Pendleton in California.

US media reported that the incident involved the detonation of unexploded ordnance.

Marines spokesman Lieutenant Ryan Finnegan said the cause of the accident was under investigation.

The deaths follow a mortar explosion in March in which seven marines were killed during a live-fire training exercise in Nevada.

A military investigation determined human error was to blame on that occasion.

The investigation also determined the mortar team involved in the accident had not conducted "appropriate preparatory training".