Bali, Indonesia | Indonesian warships led the hunt Thursday for a navy submarine that went missing with 53 crew aboard during regular exercises, as other nations sent vessels to help.
An oil spill where the submarine was thought to have submerged early Wednesday pointed to possible damage, and fanned fears in the Southeast Asian nation of disaster.
While the KRI Nanggala 402 may have several days' worth of oxygen reserves, defence analysts warned that the vessel could break into pieces if it had sunk to depths believed to be as much as 700 metres (2,300 feet).
Australian Defence Minister Peter Dutton said initial reports raised the prospect of "a terrible tragedy".
The German-built submarine was scheduled to conduct live torpedo exercises when it asked for permission to dive. It lost contact shortly after.
Navy spokesman Julius Widjojono said Thursday that search teams were focused on an area around the oil slick, but that the exact location of the vessel had yet to be pinpointed.
"It has not been found yet," Widjojono told AFP.
Six warships and a helicopter have been sent to look for the sub, the navy said.
– 'Very distressing' –
Other nations including the United States, Australia, France and Germany have offered help.
"It's very distressing for families and particularly for the Indonesian navy," Australian Foreign Minister Marise Payne told broadcaster ABC.
"We've indicated that we will help in any way we can."
Neighbouring Singapore and Malaysia have already dispatched ships that are expected to arrive in the coming days, including the city-state's MV Swift Rescue — a submarine rescue vessel.
The military has so far refused comment about whether the submarine, carrying 53 crew, was over capacity.
On Wednesday, the navy said the submarine might have sunk to vast depths.
"It's possible…a blackout occurred so the vessel couldn't be controlled and emergency procedures could not be launched and this caused it to go down to 600 or 700 metres," it said.
Antoine Beaussant, a French navy vice admiral, said the submarine was not built to withstand those depths.
"If it went down to rest at 700 metres the likelihood is it would have broken up," he told AFP.
Frank Owen, secretary of the Submarine Institute of Australia, also warned over the vessel's fate.
"If the submarine is on the seabed, and if it is in the depth of water that is there, there is little they can do to actually get the people out," he told Australian media.
"The only way of getting those people out would be to salvage the submarine which is a lengthy process."
– Workhorse submarine –
Indonesia, which has been moving to upgrade its ageing military equipment in recent years, has five German and South Korean-built submarines in its fleet.
The 1,300-tonne KRI Nanggala 402 was first delivered for service in 1981.
It is a Type 209 diesel-electric attack submarine that has served in more than a dozen navies around the world, including Greece, India, Argentina and Turkey, over the past half century.
While Indonesia has not previously suffered a major submarine disaster, other countries have been struck by accidents in the past.
Among them was the 2000 sinking of the Kursk, the pride of Russia's Northern Fleet.
That submarine was on manoeuvres in the Barents Sea when it sank with the loss of all 118 aboard. An inquiry found a torpedo had exploded, detonating all the others.
Most of its crew died instantly but some survived for several days — with a few keeping heart-breaking diaries written in blood to their loved ones — before suffocating.
In 2003, 70 Chinese naval officers and crew were killed, apparently suffocated, in an accident on a Ming-class submarine during exercises in 2003.
Five years later, 20 people were killed by poisonous gas when a fire extinguishing system was accidentally activated on a Russian submarine being tested in the Sea of Japan.
And in 2018, authorities found the wreckage of an Argentine submarine that had gone missing a year earlier with 44 sailors aboard.