Rescuers Embark on All-Night Race to Save Titanic Sub Crew

A massive search and rescue effort for a missing submersible near the wreck of the Titanic entered a critical stage late Wednesday, with just one night left before the oxygen supply for the five people on board runs out.

While coast guard officials insisted they remained "hopeful," with a surge of assets and experts joining the operation and sonar picking up unidentified underwater noises, the challenge of locating and recovering the crew alive appeared increasingly formidable.

"Sometimes you're in a position where you have to make a tough decision. We're not there yet," said US Coast Guard Captain Jamie Frederick, adding that it remained a search-and-rescue mission, percent."

Organisers of the multinational response -- which includes US and Canadian military planes, coast guard ships, and teleguided robots -- are focusing their efforts in the North Atlantic close to multiple "underwater noises" detected by sonar late Tuesday and Wednesday.

The sounds raised hopes that the passengers on the small tourist craft are still alive though experts have not been able to confirm their source.

"We don't know what they are, to be frank with you," said Frederick, who added: "We have to remain optimistic and hopeful."

Titan began its descent at 8:00 am on Sunday and had been due to resurface seven hours later, according to the US Coast Guard.

Rescuers, who have received help from around the world, estimate that passengers may run out of oxygen on Thursday morning, based on the sub's capacity to hold up to 96 hours of emergency air.

The 21-foot (6.5-meter) tourist craft lost communication with its mothership less than two hours into its trip to see the remains of the Titanic, which sits more than two miles (nearly four kilometers) below the surface of the North Atlantic.

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.