The deaths occurred during a periodic sweep of explosive material to make ranges safe for future exercises, said a Marine official who spoke on condition of anonymity because he was not authorized to speak publicly.
The identities of the dead were withheld pending notification of relatives. Base officials released no details on the 11 a.m. accident. Authorities were investigating the cause.
"We offer our heartfelt prayers and condolences to the families of the Marines lost today in this tragic accident," said Brig. Gen. John W. Bullard, commanding general of Marine Corps Installations West at Marine Corps Base Camp Pendleton. "Our first priority is to provide the families with the support they need during this difficult time."
The deaths come about eight months after a mortar explosion killed seven Marines during a live-fire training exercise in Nevada.
A military investigation determined human error was to blame for that accident. According to the probe's findings, a Marine operating a 60 mm mortar tube and ammunition did not follow correct procedures, resulting in the detonation of a high explosive round at the mortar position.
The investigation also determined the mortar team involved in the accident had not conducted "appropriate preparatory training."