The USS Bonhomme Richard, an amphibious assault vessel, was in port in San Diego for maintenance when the explosion erupted.

Thick smoke could be seen billowing from large sections of the ship, as scenes broadcast by CNN showed fireboats using water cannon to try to control the blaze.

The official Twitter account of Naval Surface Forces, US Pacific Fleet, said 17 sailors and four civilians were taken to the hospital for minor injuries. Several firefighters reportedly also suffered burns and smoke inhalation.

Colin Stowell, head of the San Diego fire department, told CNN that the fire could go on "for days" and "just burn down to the waterline."

The cause of the explosion was not immediately clear.

About 160 sailors were on board at the time. San Diego is the ship's home port.