Indonesia's military said it is seeking assistance from Australia, Singapore and India after a naval submarine with 53 people aboard disappeared north of the resort island of Bali on Wednesday.
"It is true that the KRI Nanggala-402 lost contact since early this morning around 3:00 a.m.," said First Admiral Julius Widjojono.
Military authorities said they lost contact with the vessel, one of five operated by the Southeast Asian nation, during training exercises off the coast of Bali.
The vessel, which has been in use since 1981, was rehearsing for a missile-firing exercise scheduled for Thursday.
Indonesian defense officials say the German-built vessel vanished in deep blue waters about 95 kilometers north of Bali shortly after the vessel had been granted clearance to dive.
The navy immediately deployed warships and a helicopter to find the missing submarine. Searchers spotted an oil slick near where it disappeared.
Indonesian media reported that the navy believes the submarine sank into a trough at a depth of 700 meters, according to The Associated Press.
The 1,300-ton naval vessel had on board 49 crew members, three gunners, and a commander whose identities are yet to be released.
Indonesia, the world's largest archipelago nation with more than 17,000 islands, has been improving its military apparatus in recent years as it faces regional maritime challenges involving Chinese vessels near the Natuna islands.
It plans to expand its fleet of submarines by three by 2024.