The KRI Nanggala-402 submarine, one of five operated by Indonesia's military, was built in the 1970s and refitted in 2012
An Indonesian navy submarine conducting a training exercise with 53 people aboard was reported missing on Wednesday.
Hadi Tjahjanto, commander of the Indonesian National Armed Forces, told Reuters that contact had been lost with the vessel early on Wednesday, and that warships were searching waters around 60 miles north of Bali, an Indonesian island.
The first admiral of Indonesia's Navy, Julius Widjojono, confirmed to AFP that the submarine could not be reached.
Reuters reported that Indonesian officials had called on nearby Australia and Singapore for assistance in the search for the KRI Nanggala-402, a German-made vessel.
The submarine, one of five operated by Indonesia's military, was built in the 1970s and refitted in 2012, according to media reports.
Indonesia's vast waters, around an archipelago of more than 17,000 islands, span more than 3,000 miles east to west, and more than 1,000 north to south.
The Indonesian military could not immediately be reached for comment, and the Australian and Singaporean militaries did not immediately respond to requests for comment.