Indonesia's navy has lost contact with a submarine carrying 53 sailors that was preparing for a weapons training exercise in waters north of Bali, a military spokesman said Wednesday.
The German-built KRI Nanggala-402, which went into service in the early 1980s, failed to relay results of the exercise as expected, Julius Widjojono said, according to Reuters.
Indonesia's defense ministry said that an aerial search had spotted an oil slick on the water near where the submarine is thought to have performed a dive.
The oil slick could be the result of damage to the submarine's fuel tank or a signal from the crew, the navy said. Two navy vessels had been dispatched to use sonar to search the area, the defense ministry said.
"It is possible that during static diving, a blackout occurred so control was lost," the Indonesian navy said in a statement, suggesting that the sub could have descended to "a depth of 600-700 meters [2,000-2,300 feet]." An official was quoted by Reuters as saying the vessel had been built to sustain a maximum of 250 meters (820 feet).
A ministry statement said requests for assistance had been sent and Australia, Singapore and India had responded.
"We are still searching in the waters of Bali, 60 miles from Bali, (for) 53 people," military chief Hadi Tjahjantohe told Reuters in a text message.
The news agency quoted the military as saying that the submarine was engaged in torpedo training at the time of the accident, but The Associated Press said it was rehearsing for a missile-firing exercise.
Contact with the vessel was lost at 4:30 a.m. on Wednesday local time (5:30 p.m. Tuesday ET), he said.
The military chief told Indonesia's Kompas media that contact was lost after the submarine had been given permission to dive.
The Indonesian military sent out a distress call to the International Submarine Escape and Rescue Liaison Office at about 9:37 a.m. local time to report the boat missing and presumed sunk, according to Jane's.
Representatives of the defense departments of Australia and Singapore did not immediately respond to requests for comment, Reuters said.
The Indonesian navy has sent several vessels to the search area, Jane's reported.
Indonesia operates a fleet of five military submarines, including two built in Germany and three newer South Korean-built boats, according to Reuters. It plans to operate at least eight by 2024.