Fears were growing for the lives of 53 people on Wednesday after Indonesia’s defence ministry found oil floating in the sea above a deep-sea trench where a Navy submarine has vanished.
The submarine is thought to have sunk into a trough 700 metres (2,300 feet) deep while conducting a torpedo drill, according to local reports.
An aerial search found an oil spill near the submarine's dive location and two navy vessels with sonar capability have been deployed to assist the hunt, Indonesia's Defense Ministry said.
The 43-year-old submarine, KRI Nanggala-402, was around 60 miles north of the resort island of Bali when contact was lost.
"[The navy] is currently searching for it. We know the area but it's quite deep," First Admiral Julius Widjojono said.
Singapore and Australia, who have submarine rescue vessels, have also been asked for assistance.
The vessel had been cleared to dive as part of a training exercise before it failed to report back. A total of 49 crew members, its commander and three gunners are thought to be onboard.
"After permission was given according to the procedure, the submarine lost contact and could not be reached," the defence ministry said.
Military analyst Soleman Ponto said it is too early to determine the fate of the submarine conclusively.
"We don't know yet whether the communication equipments were broken or the submarine has sunken. We have to wait for at least three days," he said.
KRI Nanggala-402 was set to perform a missile firing exercise on Thursday with senior military officials in attendance.
The submarine was constructed in 1978 and has been in service in Indonesia since 1981, part of a fleet of five operated by the world's largest archipelago nation.
Much of Indonesia’s military equipment still in service is old and there have been deadly accidents in recent years, despite the country’s effort to modernise its defence capabilities.
An air force transport plane crashed into a mountain in 2016, killing all 13 people on board, during a training exercise in the remote region of Papua, and more than 100 people were killed in 2015 after a military transport plane crashed into a northern residential area two minutes after takeoff.
Indonesia hopes to up its submarine fleet to at least eight by 2024 as it faces growing threats to its maritime sovereignty.
Chinese vessels have posed a particular growing challenge in recent years and have been sighted numerous times near the Natuna islands.