On Sunday, the hunt for a missing Indonesian submarine with 53 crew turned into a salvage effort after recovered debris suggested it broke apart as it sank off the coast of Bali.

Warships, planes and hundreds of military personnel had led a frantic search for the KRI Nanggala 402 since it disappeared this week during training exercises, hoping for a miracle rescue before its known oxygen reserves ran out.

But the Indonesian Navy acknowledged Saturday that pieces of the submarine, including items from inside the vessel, had been retrieved.

They declared that it had sunk, effectively ending any chance of finding survivors.

Among the items recovered was a piece of the torpedo system and a bottle of grease used to lubricate periscopes.

They also found a prayer mat commonly used in the country. President Jokowi described the missing sailors as Indonesia's "best patriots".

"All Indonesians convey their deep sadness over this incident, especially to the families of the submarine crew," he said.

Authorities have not given an official explanation for the accident, but said that the submarine may have suffered a blackout and left its crew unable to resurface.

Indonesian Navy chief Adm. Yudo Margono discounted an explosion, however, saying Saturday that the evidence suggested the submarine came apart as it was crushed by water pressure at depths of more than 800 metres (2,600 feet) -- well below what the German-built Nanggala was built to withstand.

"Submarine hulls are pressurised... but when they're breached then water would come flooding inside," said Wisnu Wardhana, a maritime expert at Indonesia's Sepuluh Nopember Institute of Technology, quoted by AFP.

"Can you imagine if water with that kind of pressure hits people?"

Retired French vice-admiral Jean-Louis Vichot earlier told AFP that a submarine's steel shell could break "like a folding accordion" when it hits depths way beyond its limits.

On Sunday, the search team focused on pinpointing the sub's exact location.

Authorities have warned that any salvage operation would be risky and difficult in the deep waters.

Singapore's MV Swift Rescue -- a submarine rescue vessel -- has arrived to aid in the recovery effort, the navy said Sunday.

Neighbouring Malaysia, as well as the United States, India and Australia, were among the nations helping in the search.

Search vessels, reconnaissance aircraft and submarine rescue ships have been deployed to scour a zone of about 10 square nautical miles (34 square kilometres).