The sailors aboard the USS Bonhomme Richard had "minor injuries" from the fire and were taken to a hospital, Lt. Cmdr. Patricia Kreuzberger said. Credit: Denis Poroy/AFP.

An explosion and fire on board the USS Bonhomme Richard at US Naval Base San Diego continues to burn for a fifth straight day Thursday, as firefighting teams inched their way deeper into its compartments in a painstaking search to find every smouldering hot spot.

Hundreds of sailors were fighting a desperate battle to keep flames away from a million gallons (3.7 million liters) of oil on board the ship, which began to list to one side.

The US Coast Guard 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 traffic within 1 nautical mile of the ship and flights over the vessel, Associated Press reported.

Officials said fire temperatures had reached up to 1,000 degrees at one point, causing the mast of the ship to collapse and threatening the central control island, raising fears that the mini aircraft carrier may be damaged beyond repair.

“All shipboard fires are difficult to fight,” said maritime lawyer Rod Sullivan, who served in the Navy. “It’s very difficult to choke off oxygen in open deck spaces,” and then to follow the flames into all the nooks on a boat, NBC San Diego reported.

It’s not uncommon for ship fires to take days to extinguish, he added, pointing to a fire last month on a car-carrying cargo ship that burned in Jacksonville, Florida, for eight days.

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 disaster, AP reported.

“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.

Even spraying water on a ship fire can be risky, said Brennan. If any aluminum on board had melted on plywood the combination could create aluminum carbide, which, in turn, can generate a flammable methane when sprayed with water.

“An uncontrollable fire like this one is among sailors’ worst fears,” he said, adding that’s why ships are designed to have so many compartments that can be closed off quickly with airtight doors.

Meanwhile, acrid smoke from the blaze wafted across San Diego and health officials urged people to stay indoors if they smelled it, AP reported.

“Seventeen sailors and four civilians are being treated for non-life threatening injuries at a local hospital,” US Navy officials said.

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

The three-alarm fire is believed to have started below those spaces, in the lower cargo hold of the ship, known as the “Deep V.”

It is a “huge open area where you store a lot of (Marine Corps) equipment and everything else,” Rear Adm. Philip Sobeck, commander of Expeditionary Strike Group 3, told reporters. “That’s where we believe it was started.”

MH-60 Sea Hawk helicopters from Helicopter Sea Combat Squadron Three have conducted continuous water bucket drops, which helped cool the ship’s flight deck and superstructure, CNN reported.

According to the LA Times, two other ships, the guided-missile destroyers USS Fitzgerald and USS Russell, were moored near the Bonhomme Richard but were moved early Sunday afternoon to berths farther from the burning vessel.

The ship was in dry dock at General Dynamics National Steel & Shipbuilding Co. in San Diego last year and has been undergoing further maintenance pierside at Naval Base San Diego. Its last deployment was in 2018.

Amphibious assault ships are used to deploy Marines in amphibious landings. The ships conduct flight operations with helicopters and jet aircraft, such as the AV-8B Harrier and its replacement, the F-35B Lightning.

According to Defense News, experts say the loss of Bonhomme Richard, whether a total loss or just lost for extensive repairs, deals a significant blow to the Navy’s plans to have F-35Bs continually deployed in the Pacific. 

“It’s a big problem, considering the F-35B is the Department of the Navy’s only fielded and deployable 5th Generation generation fighter.” said Bryan Clark, a retired submarine officer and senior fellow at the Hudson Institute.

“Only half of [our 10 amphibious assault ships] are able to carry F-35B and the Marines are looking to reduce their land-based squadrons. So the loss of Bonhomme Richard will impact the Navy’s ability to provide Combatant Commanders sea-based F-35s not subject to host-nation approval.”

Jerry Hendrix, a retired Navy captain and analyst with Telemus Group, agreed, saying that the Navy’s posture in the Pacific is going to be challenged.

“It has a huge impact,” said Hendrix, a retired Navy captain and analyst with Telemus Group. “Bonhomme Richard has been in this overhaul for two years getting these upgrades to operate F-35Bs. She has about eight more years of life left in the hull, and so she was a central cog in our Pacific operational deployment plan for the next eight-to-10 years.”