SAN DIEGO (AP) — Hundreds of sailors are fighting to save a Navy warship ravaged by fire while the military prepares for the worst: The USS Bonhomme Richard going down in flames, and its million gallons of oil on board spilling into San Diego harbor, resulting in a major environmental disaster.

The U.S. Coast Guard has hired an oil clean-up crew to put in place a containment boom that could be ready if any oil is spilled. It also halted boat and air traffic within a nautical mile of the vessel.

On Monday, health officials warned people to stay indoors as acrid smoke wafted across San Diego from one of the Navy’s worst shipyard fires in recent years. At least 59 people, including 36 sailors and 23 civilians, have been treated for heat exhaustion, smoke inhalation and minor injuries. Five people who had been in a hospital under observation were released.

Some 400 sailors along with Navy helicopters and local and federal firefighters poured water on the carrier-like ship, which erupted in flames Sunday morning. Boats with water cannons directed streams of seawater into the ship.

Rear Adm. Philip Sobeck said fire temperatures had reached up to 1,000 degrees, causing the mast of the ship to collapse and threatening the central control island where the captain operates the vessel. He said there were about two decks between the fire and the fuel supplies on board.

Water being dumped on the vessel to douse the flames was causing the 840-foot (255-meter) ship to list to one side, but crews were pumping off the water.

Sobeck said it was too soon to give up on saving the 23-year-old amphibious assault ship, which has been docked in San Diego since 2018 undergoing maintenance.

“I feel absolutely hopeful because we have sailors giving it their all,” said Sobeck, commander of Expeditionary Strike Group 3.

The fire was first reported in a lower cargo area where seafaring tanks and landing craft are parked. It appears to have started in a spot where cardboard boxes, rags and other ship maintenance supplies were being stored, Sobeck said.

A fire suppression system had been turned off because it was being worked on as part of the ongoing maintenance. The system uses Halon, a liquefied, compressed gas that disrupts a fire and stops its spread by cutting off its oxygen.

All 160 people on board were evacuated after an explosion caused by pressure from the heat, Sobeck said.

Sobeck said there was no ordnance on board the ship and he did not believe there was anything toxic.

However, the flames were burning plastic, cabling and other materials, sending a haze over downtown San Diego. The San Diego Air Pollution Control District warned that concentrations of fine particulate matter could reach unhealthful levels in some areas and that people should avoid exercising outdoors and stay indoors if possible to limit exposure.

Retired Navy Capt. Lawrence B. Brennan, a professor of international maritime law at Fordham University in New York, said there is a risk of the hull rupturing, which could cause the ship to spill its oil and leave the Navy looking at a major environmental mess.

“If this is a million gallons of oil that ends up settling on the bottom of the San Diego harbor and can’t be removed safely, we’re talking about billions of dollars of environmental damage,” said Brennan, who has investigated and litigated hundreds of maritime cases.

The ship can be used to deploy thousands of Marines to shore and has the capacity to accommodate helicopters, certain types of short-takeoff airplanes, small boats and amphibious vehicles.

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AP writers Christopher Weber and John Antczak contributed from Los Angeles.