An explosion and fire onboard the US Navy’s Wasp-class amphibious assault ship USS Bonhomme Richard (LHD-6) has left 21 people injured.

The ship was docked pierside at Naval Base San Diego in California when the fire escalated overnight.

The origin of the fire remains unknown and an investigation is pending.

Injured people include 17 US Navy sailors and four civilians. They are undergoing treatment for minor injuries and have reported being stable.

The number of injured people has increased since the first report.

A statement issued by Naval Surface Force Pacific Public Affairs said: “Firefighting teams continue operations aboard the ship.

“Fifty-seven personnel (34 Sailors and 23 civilians) have been treated for minor injuries including heat exhaustion and smoke inhalation.

“Of those, five remain hospitalised for observation and are currently in stable condition.”

USS Bonhomme Richard is in San Diego for regular scheduled maintenance availability. Around 160 sailors were onboard at the time of fire.

It further added: “All crew members have been accounted for and firefighting efforts continue aboard the ship.”

Two helicopters from Helicopter Sea Combat Squadron 3 are also conducting aerial firefighting operations.

Federal Fire San Diego Division chief Rob Bondurant said: “Currently there are two firefighting teams fighting the fire aboard the ship.

“Federal Fire is rotating their crews aboard the ship with US Navy firefighting crews from the waterfront to fight the fire in order to, find the seat of the fire and extinguish it.

“Also, Navy Region Southwest tugs are also continuously combating the fire from the bay.”