JAKARTA: The sailors aboard a missing Indonesian submarine have enough oxygen to last until Saturday, aboard a vessel that had been in good condition, the country's defence chiefs said on Thursday, as the search for the submarine continued.

There were no signs of the KRI Nanggala-402, with 53 crew aboard as of Thursday afternoon, where a search and rescue mission was ongoing in calm weather conditions.

Yudo Margono, the navy chief of staff, said that there would be enough oxygen for the sailors to last until Saturday, and that the submarine had been cleared for use.

"The submarine has received a letter of feasibility from the navy," he told a news conference.

"It was ready for battle."

He was speaking alongside the chief of Indonesia's military and its defence minister, Prabowo Subianto, at a news conference in Bali a day after the 44-year-old submarine went missing while conducting a torpedo drill north of the island.

The 1,395-tonne vessel was built in Germany in 1977, according to the defence ministry, and joined the Indonesian fleet in 1981. It underwent a two-year refit in South Korea that was completed in 2012.

Prabowo acknowledged it was "imperative that we should modernise our defence equipment faster". He did not suggest there were any problems with the vessel.

An aerial search found an oil spill near the submarine's dive location, and two navy vessels with sonar capability had been deployed to assist in the search, officials said.

Yudo also said authorities had found an item with "high magnetic force" floating at a depth of 50 to 100 metres.

Earlier, navy spokesman Julius Widjojono told KompasTV that the diesel-powered submarine that runs on electric batteries while submerged could sustain a depth of 250-500 metres (820-1640 ft).

"Anything more than that can be pretty fatal, dangerous," the spokesman told KompasTV.

The seas in the area are shallower than in other parts of the archipelago but can still reach depths of more than 1,500 metres.

In a statement on Wednesday, the navy said "it was possible that during static diving, a blackout occurred so control was lost and emergency procedures cannot be carried out and the ship falls to a depth of 600-700 metres."

Indonesia said a number of countries in the region had responded to requests for assistance.

Australian Foreign Minister Marise Payne said Australia would "help in any way we can", while Singapore has deployed a submarine rescue vessel to help, the city-state's defence minister said, and Malaysia was also sending a ship.

Indonesian military chief Hadi Tjahjanto told Reuters in a text message on Wednesday that contact with the vessel was lost at 4:30 a.m. and a search was underway 60 miles (96 km) off Bali.

The oil slick could indicate damage to the vessel or could be a signal from the crew, the navy said.

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.

Indonesia has been seeking to modernise its defence capabilities but some of its equipment is old and there have been fatal accidents in recent years.