Indonesia’s navy is searching for a submarine that went missing north of the resort island of Bali with 53 people onboard. The country’s military chief, Hadi Tjahjanto, said on Wednesday that the KRI Nanggala 402 was participating in a training exercise when it missed a scheduled reporting call. The vessel is believed to have disappeared in waters about 60 miles (95km) north of Bali, he said. Tjahjanto said the navy had deployed scores of ships to search the area, including a hydrographic survey ship, and had asked for help from Singapore and Australia, which have submarine rescue vessels. The navy 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. It said it believes the submarine sank to a depth of 600-700 meters (2,000-2,300 feet). French navy vice admiral Antoine Beaussant told Agence France-Presse the submarine would have a safety descent level of 250 metres, and “if it went down to rest at 700 metres the likelihood is it would have broken up”. The defence ministry said the submarine lost contact after being granted clearance to dive. It said a helicopter later spotted an oil spill near the dive’s starting position. The ministry said the submarine was carrying 49 crew members, its commander and three gunners. Indonesia has sought help from Australia and Singapore. “This is very distressing,” the Australian foreign affairs minister, Marise Payne, said on Thursday. “We will help our neighbours in any way we can.” 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. Tjahjanto and other military leaders were to attend. Indonesia has a fleet of five submarines and plans to operate at least eight by 2024. It is the world’s largest archipelago nation, with more than 17,000 islands, and has faced growing challenges to its maritime claims in recent years, including numerous incidents involving Chinese vessels near the Natuna islands. The 1,300-tonne KRI Nanggala 402 is a Type 209 diesel-electric attack submarine that was built in Germany. The same model has served in more than a dozen navies around the world over the past half-century. Its construction began in 1978 and Indonesia took delivery in October 1981. The KRI Nanggala had undergone several upgrades. It was refitted in 1989 in Germany and then in 2012 in South Korea, with part of its structure replaced and upgrades to its propulsion, sonar and weapons systems. The incident comes at a tense time for Indonesian security forces. Last year the president, Joko Widodo, reaffirmed the country’s sovereignty during a visit to the group of islands at the edge of the South China Sea. His visit came a week after a Chinese foreign ministry spokesperson, Geng Shuang, insisted Chinese fishers were free to conduct activities in the areas China claims as its traditional fishing grounds, which partly overlap Indonesia’s exclusive economic zone. 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 coastguard vessels, reportedly made more aggressive moves in the area and ignored Indonesian warnings to leave.