Rescue teams racing against time Saturday to save 53 Indonesia sailors aboard a sunken submarine now fear the boat has descended to depths that will crush the hull.
The "Nanggala" disappeared Wednesday during exercises off Bali.
Australia, India, Malaysia and Singapore have sent assistance. A U.S. Navy aircraft landed in Bali early Saturday to help with the search.
"Indonesia is a good friend and strategic partner. We were all deeply saddened to see the reports about their submarine and our thoughts and our prayers are with the Indonesian sailors, the Indonesian navy, and of course all their families," said Pentagon spokesman John Kirby.
An oil slick where it was thought to have submerged suggested damage to a fuel tank may have been a factor.
The Indonesia military said it had detected signs of an object at a depth of between 50 meters and 100 meters and had deployed ships with sonar-tracking equipment.
Its navy has deployed search helicopters to where contact was lost with the 44-year-old submarine.
Australia has deployed a sonar-equipped frigate with a helicopter.
"The possibility of it having fallen underneath its maximum diving depth thereby leading to the implosion of the submarine will have to be considered," said Collin Koh, Research Fellow at the Institute of Defense and Strategic Studies, according to a Reuters report. "So there's a possibility...oxygen might have already run out," said Koh.
If the submarine was still intact, Indonesian officials said Friday it would only have enough air to last until around dawn Saturday if equipment was functioning properly.
The Bali Sea can reach depths of more than 1,500 meters.
