The search for the submarine is concentrated around an oil spill on the sea surface that was discovered in the area of ​​KRI Nanggala's latest known position when it had a surface position.

However, it is unclear whether the discharge comes from the submarine.

This may indicate that the submarine has been damaged.

It could also be a signal from the crew, according to the Indonesian Navy.

President Joko Widodo said on Thursday that the search and rescue effort has the highest priority for the country.

Torpedo exercise

The German-made diesel-powered submarine KRI Nanggala was to carry out what is described as a sharp torpedo exercise, north of the island of Bali on Wednesday.

The submarine took underwater position after it received a permit and has since had no contact with the outside world.

It was reported missing after the crew failed to respond to a scheduled contact with the naval staff that would have taken place early on Wednesday morning, local time.

Subsequently, the submarine was reported missing, according to British Janes Defense.

"Hopefully we can save them before the air runs out," said Admiral Yudo Margono, chief of staff for the Navy.

The submarine's air reserve in the event of a "blackout" is 74 hours, he said.

This means that the crew has enough fresh air until 3 o'clock on Saturday morning local time, reports the news agency AP and Jakarta Post.

The admiral said that the submarine was in good condition and ready for battle.

Operative since 1981

The more than 40-year-old submarine has undergone some modernisations and became operational in 1981.

Indonesia is said to have sent six warships and a helicopter and has requested support from Singapore and Australia.