JAKARTA, Indonesia  — Indonesia’s navy is searching for a submarine that went missing north of the resort island of Bali with 53 people on board, the military said Wednesday.

Military chief Hadi Tjahjanto said the KRI Nanggala 402 was participating in a training exercise when it missed a scheduled reporting call.

Indonesian navy submarine KRI Nanggala sails off Tuban, East Java, in 2014 Eric Ireng/Associated Press

The submarine is believed to have disappeared about 60 miles north of Bali, he said.

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.

The Defense 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.

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  2,000-2,300 feet.

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 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.

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.

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.