Rescue teams have been racing against time to find a missing Indonesian naval submarine and its 53 crew before oxygen runs out.
There are fears it may already have sunk to a depth where it might have been crushed.
The KRI Nanggala 402 disappeared on Wednesday during exercises off the coast of Bali, sparking a frantic search to locate the stricken vessel.
Australia, India, Malaysia and Singapore have so far sent assistance.
The US military also said earlier that it would send a P-8 Poseidon aircraft to help the search.
An oil slick where it was thought to have submerged suggested damage to a fuel tank may have been a factor.
Pentagon spokesman John Kirby said the US was “deeply saddened” by the turn of events.
“Our thoughts are with the Indonesian sailors and their families,” Mr Kirby said in a statement. “At the invitation of the Indonesian government, we are sending airborne assets to assist in the search for the missing submarine.”
The Indonesian military said late on Thursday night that it had detected signs of an object at a depth of between 50 and 100 metres (165 to 330 feet), and had deployed ships with sonar-tracking equipment in the hope it was the KRI Nanggala 402.
A military spokesman said the submarine would only have enough oxygen to last until around 03:00 local time on Saturday (21:00 GMT on Friday). That deadline has now passed.
But early on Saturday morning, a navy official, Julius Widjono, told CNN Indonesia that he was optimistic that the vessel would still be found.
“We must be optimistic. We ask for prayers from all Indonesians that 53 people will be safe,” Mr Widjono said.
At least six warships, a helicopter and 400 people have since been involved in the search. Singapore and Malaysia have dispatched ships to the area, and Australia, France and Germany have also offered assistance.
“We keep waiting, we keep praying,” Ratih Wardhani, whose brother Major Wisnu Subiyantoro among the crew, told the BBC.
“We hope that God will ease the effort of the joint rescue team from the Indonesian Navy and other countries, and they will successfully bring the Nanggala submarine with its 53 personnel to reunite with their families happily,” she said.
“That is our hope. We can only support them with our prayers and our optimism that they will return.”
The KRI Nanggala 402 lost contact shortly after requesting permission to dive during live torpedo exercises early on Wednesday morning. The German-built vessel is one of five submarines operated by Indonesia. It was made in the late 1970s, and underwent a two-year refit in South Korea that was completed in 2012.
A navy spokesman told the BBC the incident was the first time Indonesia had lost one of its submarines. But similar incidents have happened elsewhere.
In 2000, the Kursk, a Russian navy sub, sank during manoeuvres in the Barents Sea with the loss of all 118 crew. An inquiry found that one torpedo had exploded, detonating all the others. Most of the Kursk’s crew died instantly but some survived for several days before suffocating.
In 2003, 70 Chinese naval officers and crew were killed in an accident on a Ming-class submarine during exercises.
And in 2017, an Argentine military submarine went missing in the southern Atlantic with 44 crew on board. Its wreck was located a year later and officials said the submarine had imploded.