An explosion on a US naval ship injured 21 people on Sunday, a forces spokesman confirmed.
The blast, which also caused a fire, took place on the USS Bonhomme Richard, at a San Diego base shortly before 9am local time.
Seventeen sailors and four civilians were hospitalised with “non-life-threatening injuries", Mike Raney, a spokesman for Naval Surface Force, US Pacific Fleet said.
It was not immediately known where on the 840ft amphibious assault vessel the blast occurred but the cause of the fire is under investigation.
The Bonhomme Richard was undergoing routine maintenance at the time and flames sent up a huge plume of dark smoke visible around San Diego.
Around 160 sailors and officers were on board, Mr Raney said, which is far fewer than the thousand typically on the ship when it is on active duty.
The ship has the capacity to deploy and land helicopters, smaller boats and amphibious vehicles.
Two other docked ships, USS Fitzgerald and USS Russell, were moved to berths away from the fire, Mr Raney said.