Photos: 21 injured in fire aboard USS Bonhomme Richard at Naval Base San Diego

Port of San Diego Harbour Police Department boats combat a fire aboard the amphibious assault ship USS Bonhomme Richard (LHD 6) at Naval Base San Diego, California, on July 12, 2020.

Twenty-one people suffered minor injuries in an explosion and fire Sunday on board a ship at Naval Base San Diego, military officials said.

The blaze was reported shortly before 9 am on USS Bonhomme Richard, said Mike Raney, a spokesman for Naval Surface Force, US Pacific Fleet.

Seventeen sailors and four civilians were hospitalised with "non-life threatening injuries," Raney said in a brief statement. He didn't provide additional details.

The cause of the fire was under investigation. It wasn't immediately known where on the 255-meter amphibious assault vessel the blast and the fire occurred. The flames sent up a huge plume of dark smoke visible around San Diego.

Rear Adm. Philip Sobeck, commander of Expeditionary Strike Group 3, told the San Diego Union-Tribune that the Navy thinks the fire began somewhere in a lower cargo hold where marine equipment and vehicles are stored.

The fire was not a fueled by fuel oil, hazardous materials or electrical causes, Sobeck told the newspaper. It was fueled by paper, cloth, rags or other materials in a standard fire. He said he is not concerned about the air quality or toxicity around the fire.

There was no ordnance on board, Sobeck said. There are a million gallons of fuel on board, he said, but that is "well below" any heat source.

The explosion was probably caused by a change in air pressure, he told the Union-Tribune.

San Diego is the Bonhomme Richard's home port, and the ship was undergoing routine maintenance at the time of the fire. About 160 sailors and officers were on board, Raney said - far fewer than the thousand typically on the ship when it's on active duty.

All crew members were accounted for, said Admiral Mike Gilday, Chief of Naval Operations.

"We are grateful for the quick and immediate response of local, base, and shipboard firefighters aboard USS Bonhomme Richard," Gilday said in a statement.

The 23-year-old ship has the capacity to deploy and land helicopters, smaller boats and amphibious vehicles. Because of its age, a fire could be particularly destructive, especially if it reached the engine room and other tight spaces with machinery, said Lawrence B. Brennan, a professor of admiralty and international maritime law at Fordham University in New York.

"The heat of a fire of this nature can warp the steel, and that can be a major problem for any ship," said Brennan. "On an older ship, it's even more of a problem."

Two other docked ships, USS Fitzgerald and USS Russell, were moved to berths away from the fire, officials said.

Smoke rises from a fire onboard the U.S. Navy amphibious assault ship USS Bonhomme Richard at Naval Base San Diego, California.