The oxygen supply for the 53 crew members of an Indonesian submarine missing in waters off Bali is believed to have run out early Saturday with no sign of the vessel while the search resumed, bolstered by the arrival of a sonar-equipped Australian warship.

The KRI Nanggala 402 went missing after its last reported dive Wednesday off the resort island, and concern is mounting it may have sunk too deep to reach or recover in time. The navy chief said the submarine was expected to run out of oxygen early Saturday morning.

“We will maximize the effort today, until the time limit tomorrow at 3 a.m.,” military spokesperson Maj. Gen. Achmad Riad told reporters Friday. A news conference was scheduled for later Saturday morning.

There have been no signs of life from the submarine, but family members held out hope that the massive search effort would find the vessel in time.

“The family is in a good condition and keeps praying,” said Ratih Wardhani, the sister of 49-year-old crewman Wisnu Subiyantoro. “We are optimistic that the Nanggala can be rescued with all the crew.”

Twenty-four Indonesian ships and a patrol plane were mobilized for the search, focusing on the area where an oil slick was found after the submarine disappeared during an exercise. Rescuers made similar massive searches in the previous two days.

Also read: Search for Indonesia submarine focuses on oil slick off Bali

An American reconnaissance plane, P-8 Poseidon, was expected to join the search Saturday and a second Australian ship was due soon.

“These two Australian ships will help expand the search area and extend the duration of the search effort,” Australian Navy Rear Adm. Mark Hammond said.

Singaporean and Malaysian rescue ships were also expected in the coming days.

Indonesian President Joko Widodo canceled a visit to Banyuwangi port, where some rescue ships left earlier, to prepare for a weekend regional summit in Jakarta, officials said. He asked Indonesians to pray for the crew’s safe return, while ordering all-out efforts to locate the submarine.

“Our main priority is the safety of the 53 crew members,” Widodo said in a televised address on Thursday. “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.”

Also read: Worries grow about Indonesian sub's crew as oxygen dwindles

There’s been no conclusive evidence the oil slick was from the sub. Navy Chief of Staff Adm. Yudo Margono said oil could have spilled from a crack in the submarine’s fuel tank or the crew could have released fuel and fluids to reduce the vessel’s weight so it could surface.

Margono said an unidentified object exhibiting high magnetism was located at a depth of 50 to 100 meters (165 to 330 feet) and officials held out hope it is the submarine.

The navy said it believes, however, that the submarine sank to a depth of 600-700 meters (2,000-2,300 feet), much deeper than its collapse depth, at which water pressure would be greater than the hull could withstand. The vessel’s collapse depth was estimated at 200 meters (655 feet) by a South Korean company that refitted the vessel in 2009-2012.

The cause of the disappearance is still uncertain. The navy has said an electrical failure could have left the submarine unable to execute emergency procedures to resurface.

Submarine accidents are often disastrous.

In 2000, the Russian nuclear submarine Kursk suffered internal explosions and sank during maneuvers in the Barents Sea. Most of its 118 crew died instantly, but 23 men fled to a rear compartment before they later died, mainly of suffocation. 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).

But in 2005, seven men aboard a Russian mini-sub were rescued nearly three days after their vessel was snagged by fishing nets and cables in the Pacific Ocean. They had only six hours of oxygen left before reaching the surface.

The German-built diesel-powered KRI Nanggala 402 has been in service in Indonesia since 1981 and was carrying 49 crew members and three gunners as well as its commander, the Indonesian Defense Ministry 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.