"With the authentic evidence we found believed to be from the submarine, we have now moved from the sub miss phase to sub sunk", Admiral Margono said.
Hundreds of military personnel were battling against time to find the German-built KRI Nanggala 402, which disappeared in the Bali Sea on Wednesday as it prepared to conduct a torpedo drill.
The Indonesian navy submarine KRI Alugoro sails during a search for KRI Nanggala, another submarine that went missing while participating in a training exercise on Wednesday, in the waters off Bali Island, Indonesia, Thursday, April 22, 2021.
Officials have said the air supply of the submarine, which had been cleared for use and was said to be in good condition, would last only until Saturday.
The U.S. will "do everything possible to support Indonesia's search and rescue effort", National Security Adviser Jake Sullivan told Foreign Minister Retno Marsudi in a phone call on Friday, according to a U.S. National Security Council spokesperson.
Debris found on Saturday was confirmed to have come from the sub, navy officials said.
Two Australian Navy ships were heading for the search area including a frigate with special sonar capabilities, the defence department said.
While nothing conclusive had been found in the search so far, Yudo said an item with "high magnetic force" at a depth of 50-100 metres (164-328 ft) had been spotted.
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Neighbouring Singapore and Malaysia, as well as the United States and Australia, were among nations helping in the hunt with almost two dozen ships deployed to scour a search zone covering about 34 square kilometres (10 square nautical miles).
Singaporean rescue ships are also expected later Saturday, while Malaysian rescue vessels were due to arrive Sunday, bolstering the underwater hunt, Whimbo said. He asked Indonesians to pray for the crew's safe return, while ordering all-out efforts to locate the submarine. Margono said the oil could have spilled from a crack in the vessel's fuel tank, or the crew could have released the fuel to reduce the submarine's weight in order to surface.
Not only is that depth beyond the maximum depth of the submarine, potentially putting it at risk of a catastrophic hull collapse, but it also possibly puts it out of reach of available recovery options.
The cause of the disappearance is still uncertain.
One of the people on board was the commander of the Indonesian submarine fleet, Harry Setiawan.
That submarine was on manoeuvres in the Barents Sea when it sank with the loss of all 118 aboard.
And in 2018, authorities found the wreckage of an Argentine submarine that had gone missing a year earlier with 44 sailors aboard.
Indonesia, 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.