The 53 sailors on board a missing Indonesian submarine only have enough oxygen to last them for two more days.

The country’s navy has been searching for the vessel ever since it went missing during a training exercise in waters north of the island of Bali.

The 43-year-old sub was conducting a torpedo drill on Wednesday but failed to relay the results as expected, a navy spokesman said.

And now, Yudo Margono, the navy chief of staff, has warned there is only enough oxygen for the sailors to last until Saturday.

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.

Indonesian President Joko Widodo invited people to pray that “the sub is found and the crew is safe”.

“The military, navy and national rescue agency are working together with other institutions on the search and rescue.

“Our priority is the safety of the 53 crew,” he said.

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

But the navy said it was also possible that “during static diving, a blackout occurred so control was lost,” which would mean, “emergency procedures cannot be carried out and the ship could fall to a depth of 600-700 metres”.

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

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.

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.

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