Australia will provide support to Indonesia after a submarine went missing north of Bali with 53 people on board.
The KRI Nanggala 402 was participating in a training exercise when it missed a scheduled reporting call, military chief Hadi Tjahjanto said on Wednesday.
The submarine is believed to have disappeared in waters about 95 kilometres north of Bali, he said.
Commander Tjahjanto said the navy has deployed scores of ships to search the area, including a hydrographic survey ship, and has asked for help from Singapore and Australia, which have submarine rescue vessels.
Foreign Minister Marise Payne said Australia would provide whatever support it could.
Commander of the Indonesian National Armed Forces, Hadi Tjahjanto. Source: Anton Raharjo/Anadolu Agency
"We are obviously very concerned about these reports. It's very distressing for families and particularly for the Indonesia Navy," she told AM on Thursday morning.
"We've indicated that we will help in any way we can. We operate very different submarines from this one, but the Australian Defence Force and Australian Defence organisation will work with defence operations in Indonesia to determine what we may be able to do.
"We will go to the support of our neighbour in any way we can."
The Defence Ministry said the submarine lost contact after being granted clearance to dive. It said a helicopter later spotted an oil slick near the dive's starting position.
The submarine was carrying 49 crew members, its commander and three gunners, it said.
An electrical failure may have occurred during the dive, causing the submarine to lose control and become unable to undertake emergency procedures that would have allowed it to resurface, the navy said.
It said it believes the submarine sank to a depth of 600-700 metres.
The German-built submarine, which has been in service in Indonesia since 1981, was rehearsing for a missile-firing exercise that was to take place on Thursday.
Commander Tjahjanto and other military leaders were to attend.
Indonesia currently has a fleet of five submarines and plans to operate at least eight by 2024.
The country, the world's largest archipelago nation with more than 17,000 islands, has faced growing challenges to its maritime claims in recent years, including numerous incidents involving Chinese vessels near the Natuna islands.
Last year, President Joko Widodo reaffirmed the country's sovereignty during a visit to the islands at the edge of the South China Sea.
A crew of Indonesian Navy submarine KRI Nanggala as an officer walks aboard, at base of the Eastern Fleet Command in Surabaya, East Java, Indonesia. Source: AP
His visit came a week after Chinese Foreign Ministry spokesperson Geng Shuang insisted that Chinese fishermen are free to conduct activities in areas China claims as its traditional fishing grounds, which partly overlap Indonesia's exclusive economic zone.
Mr Geng's statement drew indignation in Indonesia and prompted the military to increase its forces at the islands.
Although China has been making such claims for years, recently dozens of Chinese fishing boats, escorted by coast guard vessels, have reportedly made more aggressive moves in the area and ignored Indonesian warnings to leave.
