Kirby said the US Defense Department was "deeply saddened" by the news of the lost submarine KRI Nanggala 402 that disappeared Wednesday during exercises off the coast of Bali.

"Our thoughts are with the Indonesian sailors and their families," Kirby said in a statement.

"At the invitation of the Indonesian government, we are sending airborne assets to assist in the search for the missing submarine."

Kirby said US Defense Secretary Lloyd Austin planned to speak Friday with Indonesian defense chief Prabowo Subianto "to convey our sorrow and to discuss how else the United States can be of assistance."

An oil spill where the submarine was thought to have submerged pointed to possible damage to its fuel tank, and fanned fears of a deadly disaster.

The crew of the German-built vessel could have enough oxygen until early Saturday, but time was quickly running out as rescuers scoured the coast off Bali for signs of its location.