An Australian frigate equipped with sonar systems and a submarine-hunting helicopter will reach the search area of he coast of Bali on Friday.

A support ship off the coast of Brunei will join the search early next week, replenishing other vessels with fuel, water and stores.

Despite Indonesia sending a helicopter and five ships to join the search, they hav found no signs of the KRI Nanggala-402, which went missing early on Wednesday during a torpedo drill.

“I have ordered the military chief, navy chief of staff, the search and rescue agency and other instances to deploy all the forces and the most optimal efforts to find and rescue the submarine crew,” President Joko Widodo said.

“The main priority is the safety of the 53 crew members.”

Yudo Margono, the navy chief of staff, said the search was being aided by calm conditions but the crew’s air supply would last only until the early hours of Saturday.

“Hopefully before they can be found, the oxygen will be enough,” he told a news conference in Bali.

The vessel had been cleared for use and was in good condition, he added.

The 1395-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.

Malaysia and Singapore are also sending ships to join the search, while the United States had despatched “airborne assets” to help find the missing sub.

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.

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

Chief of staff Yudo said authorities had found an item with “high magnetic force” floating at a depth of 50 to 100 metres.

Navy spokesman Julius Widjojono told KompasTV that the diesel-powered submarine that runs on electric batteries while submerged could sustain a depth of 250-500m.

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

The waters in the area are shallower than in other parts of the archipelago but can still reach depths of more than 1500m.

The navy on Wednesday said a blackout may have occurred during static diving, causing a loss of control and preventing emergency procedures from being carried out if the vessel falls to a depth of 600-700m.

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

It now operates five – the two German-built Type 209 submarines and three newer South Korean vessels.

Fleet commander Mark Hammond said the two Australian ships would help expand the search area and extend the duration of the effort.

“My thoughts are with the submariners of KRI Nanggala, their families and the Indonesian people,” Rear Admiral Hammond said.

“As always, we stand ready to assist our fellow mariners in the Indonesian Navy.”

The Australian Defence Force remains in close contact with Indonesia to determine if it can provide any further assistance.

-REUTERS