A missing Indonesian navy submarine with 53 people aboard is set to run out of oxygen in three days, Indonesian authorities said Thursday, as rescuers continued a frantic search for the vessel.

The submarine was conducting a training exercise when it disappeared in the waters north of Bali, an Indonesian island, early Wednesday. Rescuers found an oil spill while searching for the vessel, which could have been a signal from the crew or an indication of equipment failure.

The search party has grown to include six warships, a helicopter and about 400 people, according to BBC News.

Rescue ships from Malaysia and Singapore are on the way, and Indonesian military officials say that Australia, the United States, Germany, France, Russia, India and Turkey have all offered help. India’s defense minister said Thursday that he had ordered the country’s navy to send a deep submergence rescue vessel to Indonesia.

But rescue teams may not be capable of locating the missing vessel. Frank Owen, secretary of the Submarine Institute of Australia, told the Associated Press that most rescue systems cannot operate at depths below 600 meters. Indonesian authorities believe that the submarine sank to a depth of 600 to 700 meters.

The submarine, one of five operated by Indonesia’s military, was built in the 1970s and refitted in 2012, according to media reports. Authorities have said it fired off two torpedoes in the Bali Strait as part of a war simulation, then failed to resurface and lost contact.

Indonesia is the world’s fourth-largest country by population, and its vast waters, around an archipelago of more than 17,000 islands, span more than 3,000 miles east to west, and more than 1,000 north to south.

It is unusual for a naval submarine to go missing.

In 2017, a submarine carrying 44 people disappeared off the coast of Argentina, prompting international search and rescue efforts. The Argentine navy confirmed soon after that an “anomalous, singular, short, violent and nonnuclear event, consistent with an explosion, was registered” near where the ARA San Juan had last been located.

The rescue mission was eventually called off, and the wreckage wasn’t found for another year.

In 2000, a Russian submarine carrying 118 people disappeared in the Barents Sea, and Russian officials later said torpedoes had exploded on board. A note recovered from the ship suggested at least 23 people may have initially survived but were not rescued in time. There were no survivors.

There have been several high-profile sea searches in Indonesia in recent years.

In January, search-and-rescue operations were launched in the Java Sea after a Boeing 737-500 carrying 62 people crashed shortly after takeoff from the capital of Jakarta. There were no survivors.

In 2018, a Boeing 737 Max flown by Indonesian airline Lion Air crashed into the Java Sea shortly after takeoff. All passengers and crew were killed, and the incident came under further scrutiny a few months later when another 737 Max, flown by Ethiopian Airlines, crashed and killed everyone on board. All 737 Max airliners were then grounded for a lengthy investigation.

Four years earlier, an AirAsia Airbus A320 also crashed into the Java Sea after taking off from Jakarta.

This report has been updated.