Search for the missing Indonesian Navy submarine lost in the Bali Sea with 53 crew has been intensified, as the US joins the rescue teams from several countries, who are battling against time to find them. According to experts, the crew could face oxygen shortage if not rescued early.

The 44-year-old KRI Nanggala-402 lost contact on Wednesday as it prepared to conduct a torpedo drill. Search helicopters and navy ships left Bali and a naval base in Java at first light heading to the area where contact was lost with the submarine.

President Joko Widodo said late on Thursday said "The main priority is the safety of the 53 crew members."

According to a Reuters report, Indonesia's navy said it was investigating whether the submarine lost power during a dive and could not carry out emergency procedures as it descended to a depth of 600-700 metres, well beyond its survivable limits.

The report citing Indonesian Navy Chief of Staff Yudo Margono said an object with "high magnetic force" had been spotted "floating" at a depth of 50-100 metres and an aerial search had earlier spotted an oil spill near the submarine's last location.

Officials said, "If the submarine was still intact it would only have enough air to last about another 15 hours until early Saturday morning."

Navy spokesman Julius Widjojono said the diesel-electric powered submarine could withstand a depth of up to 500 metres (1,640 ft), anything more could be fatal. The Bali Sea can reach depths of more than 1,500 metres.

An Indonesian defence expert said the crew could still be found alive. Australia, India, Malaysia, Singapore and the United States have sent specialised ships or aircraft in response to Indonesian requests for assistance.

The U.S. Defence Department is sending "airborne assets" to assist in the submarine search, a Pentagon spokesman said.

According to the report, two Australian Navy ships were heading for the search area including a frigate with special sonar capabilities, the defence department said. Indonesia operates five submarines - two German-built Type 209s including Nanggala and three newer South Korean vessels.

It has been seeking to modernise its defence capabilities but some of its equipment is old and there have been fatal accidents in recent years.