Australia has offered to help search for an Indonesian submarine lost with 53 sailors on board.
The vessel has not been seen or heard from since Wednesday, when it was participating in a training exercise north of Bali.
Indonesia has sought help from Australia and Singapore.
"We are obviously very concerned about these reports. It's very distressing for families and particularly for the Indonesian Navy," Foreign Minister Marise Payne said on Thursday.
"We operate very different submarines from this one, but the Australian Defence Force and Australian Defence Organisation will work with defence operations in Indonesia to determine what we may be able to do.
"We will go to the support of our neighbour in any way we can."
The submarine was conducting a torpedo drill when it missed a scheduled reporting call.
It is believed to have disappeared in waters about 95 kilometres north of Bali.
The submarine was carrying 49 crew members, its commander and three gunners.
The vessel lost contact after being granted clearance to dive and could have sunk to a depth of between 600 and 800 metres, well below its safe operating levels.
Defence Minister Peter Dutton described the incident as a terrible tragedy and said deep waters in the location would make a recovery very difficult.
Submarine rescue expert Frank Owen said the ADF could provide a remote operating vehicle to help map the ocean terrain.
"If it is in the depth of water that's there, then there will be little they can do to actually get the people out," he told the ABC.
"The only way to get the people out would be to salvage the submarine, and that's a lengthy process."
Independent senator Rex Patrick, a former submariner, expressed deep concern but said he believed there was still hope.
"This is a chilling reminder of just how dangerous submarine service can be," he said.
Senator Payne said the submarine search and rescue would be a complex task.
"Those submariners and their families are very much in need of all of our thoughts and prayers," she said.