JAKARTA, Indonesia (AP) — Indonesian navy ships searched Thursday for a submarine that likely sank too deep to retrieve, making survival chances for the 53 people on board slim. Authorities said oxygen in the submarine would run out by early Saturday.

The diesel-powered KRI Nanggala 402 was participating in a training exercise Wednesday when it missed a scheduled reporting call. Officials reported an oil slick and the smell of diesel fuel near the starting position of its last dive, about 96 kilometers (60 miles) north of the resort island of Bali, though there was no conclusive evidence that they were linked to the submarine.

“Hopefully we can rescue them before the oxygen has run out” at 3 a.m. on Saturday, Indonesia’s navy chief of staff, Adm. Yudo Margono, told reporters.

He said rescuers found an unidentified object with high magnetism at a depth of 50 to 100 meters (165 to 330 feet) and that officials hope it’s the submarine.

The navy believes the submarine sank to a depth of 600-700 meters (2,000-2,300 feet), much deeper than its estimated collapse depth.

Ahn Guk-hyeon, an official from South Korea’s Daewoo Shipbuilding and Marine Engineering, which refitted the vessel in 2009-2012, said the submarine would collapse if it goes deeper than around 200 meters (655 feet) because of pressure. He said his company upgraded much of the submarine’s internal structures and systems but lacks recent information about the vessel.

Frank Owen, secretary of the Submarine Institute of Australia, also said the submarine could be at too great a depth for a rescue team to operate.

“Most rescue systems are really only rated to about 600 meters (1,970 feet),” he said. “They can go deeper than that because they will have a safety margin built into the design, but the pumps and other systems that are associated with that may not have the capacity to operate. So they can survive at that depth, but not necessarily operate.”

Owen, a former submariner who developed an Australian submarine rescue system, said the Indonesian vessel was not fitted with a rescue seat around an escape hatch designed for underwater rescues. He said a rescue submarine would make a waterproof connection to a disabled submarine with a so-called skirt fitted over the recue seat so that the hatch can be opened without the disabled submarine filling with water.

Owen said the submarine could be recovered from 500 meters (1,640 feet) without any damage but couldn’t say if it would have imploded at 700 meters (2,300 feet).

In November 2017, an Argentine submarine went missing with 44 crew members in the South Atlantic, almost a year before its wreckage was found at a depth of 800 meters (2,625 feet). In 2019, a fire broke out on one of the Russian navy’s deep-sea research submersibles, killing 14 sailors.

Indonesian President Joko Widodo asked all of the country’s people to pray that the submarine and crew could be found.

“Our main priority is the safety of the 53 crew members,” Widodo said in a televised address. “To the families of the crew members, I can understand your feelings and we are doing our best to save all of the crew members on board.”

The military said more than 20 navy ships, two submarines and five aircraft were searching the area where the submarine was last detected. A hydro-oceanographic survey ship equipped with underwater detection capabilities also was on its way to the site around the oil spills.

Margono said the oil slick may have been caused by a crack in the submarine’s tank after the vessel sank.

Neighbouring countries are rushing to join the complex operation.

Rescue ships from Singapore and Malaysia are expected to arrive between Saturday and Monday. The Indonesian military said Australia, the United States, Germany, France, Russia, India and Turkey have also offered assistance. South Korea said it has also offered help.

“The news of the missing submarine is deeply concerning,” Australian Foreign Minister Marise Payne said during a visit to New Zealand. “We will provide any assistance that we can. There’s no question that submarine search and rescues are very complex.”

Australian Defence Minister Peter Dutton called the incident “a terrible tragedy.” He told Sydney Radio 2GB that the fact that the submarine is “in a very deep part of waters” makes it “very difficult for the recovery or for location.”

“Our fervent prayers and hopes go out to the crew of KRI Nanggala, for their safety and resilience,” Singapore’s Defence Minister Ng Eng Hen wrote on Facebook.

Indonesia’s navy said an electrical failure may have occurred during the dive, causing the submarine to lose control and become unable to undertake emergency procedures that would have allowed it to resurface. It was rehearsing for a missile-firing exercise on Thursday, which was eventually cancelled.

The German-built submarine, which has been in service in Indonesia since 1981, was carrying 49 crew members, its commander and three gunners, the Indonesian Defence Ministry said. It had been maintained and overhauled in Germany, Indonesia and most recently in South Korea.

More than 60 of the Type 209 class submarines have been sold and have served in 14 navies around the world, ThyssenKrupp Marine Systems spokesperson Eugen Witte said.

Indonesia, the world’s largest archipelago nation with more than 17,000 islands, has faced growing challenges to its maritime claims in recent years, including numerous incidents involving Chinese vessels near the Natuna islands.

Last year, President Widodo reaffirmed the country’s sovereignty during a visit to the islands at the edge of the South China Sea, one of the busiest sea lanes where China is embroiled in territorial disputes with its smaller neighbours.