Officials investigating after 21 sailors, civilians hospitalized in San Diego naval ship explosion

More than a dozen sailors and several civilians have been hospitalized following an explosion on a Navy ship in San Diego on Sunday, officials said.

A fire broke out aboard the USS Bonhomme Richard sometime around 8:30 a.m. local time, said officials with the Naval Surface Forces. The blaze escalated to a three-alarm fire, according to the San Diego Fire-Rescue Department, which assisted in battling the fire along with base and shipboard firefighting teams.

Seventeen sailors, as well as four civilians, are being treated at a local hospital with non-life-threatening injuries, Mike Raney, a spokesman for the Naval Surface Force, U.S. Pacific Fleet, confirmed to ABC News Sunday afternoon.

Several of the injuries were said to be serious while others were from smoke inhalation, officials said.

A defense official told ABC News that 19 federal firefighters have also suffered at least minor injuries fighting the blaze.

There were 160 sailors aboard at the time the fire started, and the entire crew is off the ship and accounted for, Naval Surface Forces said.

All SDFD responders have also been accounted for, officials said.

The USS Bonhomme Richard, currently undergoing maintenance, has a crew size of about 1,000, Raney said.

At a press conference Sunday evening, Rear Adm. Philip Sobeck said the origin of the fire appeared to be in the lower cargo hold of the ship. It was unclear what caused the explosion, but that there is a "low risk for secondary explosions," he said.

Authorities have cleared a 1,800-foot perimeter around the ship and are monitoring the temperature of the ship and the air quality, Sobeck said. The U.S. Coast Guard has also closed San Diego Channel south of Coronado Bridge to all boating traffic, he said.

Officials told ABC News it would likely be very hard to put the fire out. Local, base and shipboard firefighting teams were responding, officials said.

"All in-port ships have been contacted and directed to provide fire parties to possibly assist with firefighting efforts," Raney said.

Two firefighting teams were working on board the ship to contain the blaze, Federal Fire San Diego Division Chief Rob Bondurant said in a statement released late Sunday afternoon local time.

"Federal Fire is rotating their crews aboard the ship with U.S. Navy firefighting crews from the waterfront to fight the fire in order to find the seat of the fire and extinguish it," he said, adding that Navy Region Southwest tug boats are also battling the fire from the water.

Two neighboring Navy ships -- the USS Fitzgerald and USS Russell -- have since moved away from the fire.

The origin of the fire is pending investigation.