Indonesia's military said it was searching for a submarine with 53 crew aboard after losing contact with the vessel off the coast of Bali on Wednesday.
"The KRI Nanggala 402 lost contact early this morning," said First Admiral Julius Widjojono.
"(The navy) is currently searching for it. We know the area but it's quite deep."
Indonesia's military chief Hadi Tjahjanto told AFP neighbours Singapore and Australia had been asked for help.
The navy had dispatched warships to the area off the northern coast of Bali to search for the submarine, he said.
He added there were 53 crew aboard the vessel, which was believed to be in waters about 700 metres (2,300 feet) deep.