Indonesia's military said it was searching for a submarine with 53 crew aboard after losing contact with the vessel during naval exercises off the coast of Bali on Wednesday.
Indonesia's military chief Hadi Tjahjanto told AFP the navy had dispatched warships off the northern coast of the island to search for it, and that neighbours Singapore and Australia had been asked for help.
"Around 07:00, monitoring by a helicopter discovered an oil spill in the position where the submarine submerged," the ministry said in a statement.
He added there were 53 crew aboard the vessel, which was believed to be in waters about 700 metres (2 300 feet) deep.
First Admiral Julius Widjojono said they had lost contact with the 43-year-old German-built submarine early Wednesday morning.
"We know the area but it's quite deep," he said.
The vessel was reportedly to conduct live-fire torpedo exercises before contact was lost.
The 1 300-tonne KRI Nanggala 402 was built in 1978, according to a government website.
Indonesia, which has been moving to upgrade its military equipment in recent years, has five German and South Korean-built submarines in its fleet.