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.
A helicopter looking for the missing vessel spotted an oil spill in waters where the submarine was thought to have submerged, it added.
Some reports say contact was lost after the submarine had been given clearance to dive into deeper waters.
The navy has deployed a number of warships to search for the missing crew and officials fear the vessel sank to the bottom of a trough with a depth of 2,300ft, reports Kompas.
KRI Nanggala (402) went missing around 03:00 AM local time today, about 95 kilometers North of Bali, presumably in a 700-meters deep trench.
The navy sent out an global distress signal, and "several countries have responded and are ready to help including Singapore, Australia and India", the ministry said.
The first admiral of Indonesia's navy, Julius Widjojono, confirmed to Agence France-Presse that the submarine could not be reached.
Commander Marshal Tjahjanto will hold a media briefing to share further information about the search on Thursday from Bali, a spokesman said.
The Indonesian Navy submarine KRI Nanggala takes part in a drill off Cilegon in 2017.
"We don't know yet whether the communication equipments were broken or the submarine has sunken".
The submarine underwent a two-year refit in South Korea that was completed in 2012, according to the Indonesian cabinet secretariat's website.
It is one of five submarines owned by the Indonesian armed forces and was built in West Germany in 1978.
It plans to operate at least eight by 2024.
Indonesia has been seeking to upgrade its defense capabilities, but some of its equipment still in service is old and there have been deadly accidents involving in particular ageing military transport planes in recent years.