JAKARTA, Indonesia (AP) — Indonesia’s navy ships on Thursday were intensely searching for a submarine that likely fell too deep to retrieve, making survival chances for the 53 people on board slim. Neighboring countries rushed their rescue ships to support the complex operation.

The diesel-powered KRI Nanggala 402 was participating in a training exercise Wednesday when it missed a scheduled reporting call. Officials reported an oil slick and the smell of diesel fuel near the starting position of its last dive, about 96 kilometers (60 miles) north of the resort island of Bali, though there has been no clear evidence that they are linked to the submarine.

Indonesia’s navy said it believes the submarine sank to a depth of 600-700 meters (2,000-2,300 feet) — much deeper than its collapse depth estimated at 200 meters (656 feet) by a firm that refitted the vessel in 2009-2012.

Ahn Guk-hyeon, an official at South Korea’s Daewoo Shipbuilding and Marine Engineering, said most submarines collapse if they go deeper than 200 meters because of pressure on the hull. He said his company upgraded much of the Indonesian submarine’s internal structures and systems but it currently lacks information about the vessel because it hasn't been involved with any work on the ship in the past nine years.

Frank Owen, secretary of the Submarine Institute of Australia, also said the submarine could be at too great a depth for a rescue team to operate.

“Most rescue systems are really only rated to about 600 meters (1,969 feet)," he said. "They can go deeper than that because they will have a safety margin built into the design, but the pumps and other systems that are associated with that may not have the capacity to operate. So they can survive at that depth, but not necessarily operate.”

Indonesia’s military said that five navy ships and a helicopter were taking part in the search while a hydro-oceanographic survey ship equipped with underwater detection capabilities is on its way to the site around the oil spills.

Rescue ships from Singapore and Malaysia are expected to arrive Saturday. The military said Australia, the United States, Germany, France, Russia, India and Turkey have also offered assistance.

“The news of the missing submarine is deeply concerning,” Australian Foreign Minister Marise Payne said during a visit to New Zealand. “We will provide any assistance that we can. There’s no question that submarine search and rescues are very complex.”

Australian Defense Minister Peter Dutton called the incident “a terrible tragedy.” He told Sydney Radio 2GB. that fact that the submarine is “in a very deep part of waters” makes it “very difficult for the recovery or for location.”

Singapore's Defense Minister Ng Eng He confirmed it had dispatched its submarine rescue vessel. “Our fervent prayers and hopes go out to the crew of KRI Nanggala, for their safety and resilience,” he wrote on Facebook.

Indonesia’s 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 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.

The German-built submarine, which has been in service in Indonesia since 1981, was carrying 49 crew members, its commander and three gunners, the Indonesian Defense Ministry said. It had maintenance and overhaul in Germany, Indonesia and most recently in South Korea, from 2007 to 2012.

Indonesia currently has a fleet of five submarines and plans to operate at least eight by 2024.

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, one of the busiest sea lanes where China is embroiled in territorial disputes with its smaller neighbors.

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.

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Associated Press writers Kim Tong-hyung and Hyung-jin Kim in Seoul, South Korea, McGuirk Rod in Canberra, Australia and Nick Perry in Wellington, New Zealand contributed to this report.