More navy ships left the Indonesian base of Banyuwangi early on Friday to head to the Bali Sea where contact was lost with the KRI Nanggala-402 on Wednesday during a torpedo drill.
“The main priority is the safety of the 53 crew members,” President Joko Widodo said late on Thursday, calling for an all-out effort to find the vessel.
Helicopters also flew from Bali at first light.
Officials have said the air supply of the submarine, which had been cleared for use and was said to be in good condition, would last only until Saturday.
The diesel-electric powered submarine could withstand a depth of up to 500 metres but anything more could be fatal, a navy spokesman said. The Bali Sea can reach depths of more than 1,500 metres.
An aerial search also spotted an oil spill near the submarine’s dive location, which the navy said could indicate damage to the vessel or could be a signal from the crew.
A number of countries have responded to Indonesia’s requests for assistance, with Australia, Malaysia, India, Singapore and the United States sending specialised ships or aircraft.