The search for a missing Indonesian submarine on Thursday focused around an oil slick north of the resort island of Bali with help from Australia, Singapore and other countries, the navy said.

The KRI Nanggala 402 with 53 people on board was participating in a training exercise Wednesday when it missed a scheduled reporting call. The oil slick was spotted near the starting position of its last dive, about 96 kilometers (60 miles) north of Bali.

Indonesian navy spokesperson Julius Widjojono said the search included help from several countries, including Australia and Singapore, which have submarine rescue vessels.

The Indonesian navy has deployed scores of ships to search the area, including a hydrographic survey ship.

Australian Defense Minister Peter Dutton said he would speak to the Indonesian government. “Australia will obviously provide whatever assistance is possible and we’ll work with other partners in the region to provide whatever assistance we can,” Dutton told Sydney Radio 2GB.

“We will go to the support of our neighbor in any way we can,” Australian Foreign Minister Marise Payne told Australian Broadcasting Corp. radio.

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The submarine was carrying 49 crew members, its commander and three gunners, the Indonesian Defense Ministry 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 600-700 meters (2,000-2,300 feet).

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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. Military chief Hadi 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.