BALI, INDONESIA - A frantic search is under way for an Indonesian submarine with 53 seamen on board that disappeared during a torpedo firing drill on Wednesday (April 21).

Here is what is known about the missing vessel:

When was the submarine last seen?

The exact location of the vessel has yet to be pinpointed.

The Indonesian Army detected underwater movement at 2.5 knots around the last location of the submarine, Nanggala-402, in waters north of Bali Island, according to an official source on Thursday.

"The contact was then lost, so there is still not enough data to identify that it was a submarine," said the military spokesman, Major-General Achmad Riad.

Indonesia's Navy chief Yudo Margono said the 44-year-old submarine was conducting a torpedo drill when contact was lost. In a timeline he provided at a press conference on Thursday, he said the submarine requested permission to dive to a depth of 13m at 3am on Wednesday, as it prepared to fire a torpedo.

At 3.30am, the Nanggala-402's bow was still visible to the accompanying vessel 50m away.

However, the submarine did not respond to several calls between 3.46am and 4.46am. The periscope, which should have been visible, had also vanished.

The submarine also did not resurface as scheduled at 5.15am. A search is underway 96km off Bali.

A helicopter and two military vessels also found an oil spill and the smell of diesel fuel at several different locations, but it is not yet possible to conclude that this is submarine fuel, Major-General Riad added.

Defence analysts warned that the vessel could break into pieces if it had sunk to depths believed to be as much as 700m.

French Navy Vice-Admiral Antoine Beaussant said the submarine was not built to withstand those depths.

"If it went down to rest at 700m, the likelihood is it would have broken up," he said.

Mr 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."

What do we know about the submarine?

The German-built submarine was scheduled to conduct live torpedo exercises when it asked for permission to dive. It lost contact shortly after.

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.

The submarine is 59.5m long, 6.3m high, and 5.5m wide, with an arsenal of up to 14 Surface and Underwater Target (SUT) torpedoes.

Navy spokesman Julius Widjojono said that the submarine could sustain a depth of 250m to 500m.

Indonesia in the past operated a fleet of 12 submarines bought from the Soviet Union to patrol the waters of its sprawling archipelago.

But now it has a fleet of only five, including two German-built Type 209 submarines and three newer South Korean vessels.

How is the search being conducted?

Indonesian authorities are deploying 21 military vessels to search in the last known position of the submarine, including the underwater oceanographic vessel Rigel.

In addition, five foreign vessels - from Singapore, Malaysia, Australia and India - as well as an aircraft from the US will join the search.

The Singapore Swift Rescue vessel was expected to arrive in Bali waters on Saturday to assist the search and rescue operation, while the Rescue Mega Bakti Malaysia ship will arrive next Monday.

How is Singapore involved in the search?

The Republic of Singapore Navy (RSN) submarine rescue vessel - MV Swift Rescue - was dispatched on Wednesday afternoon to assist in the search for the missing Indonesia submarine.

In a Facebook post on Thursday, Defence Minister Ng Eng Hen said the ship was dispatched "as fast as she could get ready" after Singapore's navy chief received a request for assistance from his Indonesian counterpart.

A medical team was also added to the regular crew in the event that hyperbaric care is needed, he added.

Dr Ng also said that Singapore's military ties with Indonesia are very close, built up during bilateral exercises held over the years and with engagements at all levels.

"It is only natural that we do whatever we can to assist in times like this. The site for search operations, near Bali, is more than 1,500km away and waters are deep, which is why MV Swift Rescue sailed off as soon as she could.

"In the meantime, our fervent prayers and hopes go out to the crew of KRI Nanggala, for their safety and resilience and also to the search and locate teams of the Indonesian Navy currently on site," he added.

Singapore and Indonesia had signed a submarine rescue support and cooperation agreement in 2012 for the RSN's submarine rescue system to be made available to the Indonesian Navy, whose resources will also support the RSN in the event of a submarine disaster.

An Indonesian military spokesman was quoted in media reports as saying that the MV Swift Rescue is estimated to arrive at the search area on Saturday.

MV Swift Rescue, which has a maximum speed of 12 knots and can operate out at sea continuously for up to 28 days, is equipped with a Deep Search and Rescue Six (DSAR 6) submersible vessel.

The ship is equipped with recompression chambers, which help prevent and treat decompression sickness, an eight-bed high dependency ward and a 10-bed sick bay.

The smaller DSAR 6 submersible plays a key role in an underwater rescue. It is launched from the MV Swift Rescue and can go underwater to connect with the submarine, ferrying the crew to safety.

How long can the sailors survive underwater?

Navy chief Yudo Margono said that oxygen supplies on board the vessel would last 72 hours from the time the submarine went missing, which means that they would run out around 3am on Saturday.

However, the chances of survivors being found on the submarine are slim, a maritime expert told The Straits Times on Thursday.

The deeper the vessel is situated, the greater the pressure, Indonesia's National Maritime Institute executive director Siswanto Rusdi said.

"The best-case scenario is that the crew suffered severe injuries such as burst eardrums and blood vessels, and the worst-case scenario is that they are dead," he added.

- With additional reporting from AFP, Reuters and Xinhua.