BOSTON: Rescue workers raced to beat a rapidly closing oxygen window Wednesday as they hunt for a missing submersible near the wreck of the Titanic, after noises detected by sonar raised hopes the five people onboard are still alive.

Extra ships, specialized salvage equipment and US Navy experts arrived in the vast search area in the North Atlantic, with the passengers of the small sub estimated to have less than 24 hours of oxygen left.

Rescuers, who have received help from around the world, were concentrating efforts on trying to figure out the source of “underwater noises” detected late Tuesday by a Canadian aircraft, US Coast Guard Rear Admiral John Mauger told CBS.

“It’s something, it’s a target, it’s a focus for us to look at,” he said.

“As long as there’s an opportunity for survival, we will continue to work with this broad unified command to bring every resource to bear on the search,” Mauger added.

All communication was lost with the 21-foot (6.5-meter) tourist craft less than two hours into its descent Sunday to see the remains of the Titanic, which sits more than two miles (nearly four kilometers) below the surface of the ocean.

The submersible, named Titan, was carrying British billionaire Hamish Harding and Pakistani tycoon Shahzada Dawood and his son Suleman, who also have British citizenship. OceanGate Expeditions charges $250,000 for a seat on the sub.

Also on board is also the company’s CEO, Stockton Rush and a French submarine operator Paul-Henri Nargeolet, nicknamed “Mr Titanic” for his frequent dives at the site.