JAKARTA, Indonesia — With oxygen on a missing Indonesian submarine due to run out within hours, the chances of survival were growing increasingly slim Friday for 53 crew members aboard the vessel — if it is still intact after an underwater accident.

An international rescue effort was widening, with searchers racing against the clock in hopes of finding the KRI Nanggala-402, a diesel-electric submarine that disappeared north of Bali early Wednesday while preparing to conduct a torpedo drill.

As of Friday afternoon, Indonesian navy officials had not provided further details about the detection a day earlier of an unidentified object about 160 to 330 feet below the surface that they said had a strong magnetic field. Spokespeople for the navy did not respond to text messages and calls from The Washington Post.

The German-built submarine can withstand a depth of up to 1,640 feet, the navy has said, but it cautioned that it might have gone down in a deep spot. The Bali Sea can reach a depth of more than 5,000 feet — depths at which the vessel would not survive. Even if the submarine withstood the crushing pressure, a rescue might not be possible, experts have said.

The navy chief of staff, Adm. Yudo Margono, has said that the submarine’s oxygen supply was expected to run out by 3 a.m. local time Saturday.

In a televised address late Thursday, President Joko Widodo said he was praying for the safe return of those on board.

“To the family of the crew members, I can understand your feelings and we are doing our best to save all crew members on board,” he said.

Relatives of the missing crew members remained hopeful of a last-minute breakthrough.

“I hope that they will be found alive,” said Berda Asmara, wife of crew member Guntur Ari Prasetyo, 39, according to Reuters, adding that her husband had asked her to pray for him the last time they spoke before his vessel sailed.

Countries including Singapore, Malaysia and Australia have sent warships and underwater rescue vessels to join the search, while others have offered assistance as the window for survival has narrowed. Pentagon spokesman John Kirby said the United States was sending “airborne assets” to help track down the missing submarine.

U.S. Defense Secretary Lloyd Austin spoke with his Indonesian counterpart Friday and offered additional support, according to the Pentagon.

Indonesia’s Defense Ministry has denied suggestions that the 1,395-ton submarine, built in 1977, was not fit for service. The ministry said the vessel could operate until March 2022.

Indonesia has five submarines to patrol its vast archipelago. Two were older German-built types, and three are newer South Korean-made vessels.

David Crawshaw and Shibani Mahtani in Hong Kong contributed to this report.