BOSTON, United States (AFP)— 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.

The US Coast Guard said it had not identified whether the source of the sounds was indeed the small sub, whose passengers are estimated to have less than 20 hours of oxygen left.

“We don’t know what they are, to be frank with you,” Captain Jamie Frederick said of the underwater noises picked up late Tuesday by a Canadian aircraft, but he added: “We have to remain optimistic and hopeful.”

Extra ships, specialised salvage equipment and US Navy experts converged on the vast search area in the North Atlantic as rescuers, who have received help from around the world, concentrated their efforts near the sounds.

“We’re searching in the area where the noises were detected and we’ll continue to do so,” Frederick told reporters in Boston.

The 21-foot (6.5-metre) tourist craft lost communication with its mothership less than two hours into its descent Sunday to see the remains of the Titanic, which sits more than two miles (nearly four kilometres) 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 the company’s CEO, Stockton Rush, and a French submarine operator Paul-Henri Nargeolet, nicknamed “Mr Titanic” for his frequent dives at the site.

Ships and planes have scoured 10,000 square miles (around 20,000 square kilometres) of surface water — roughly the size of Massachusetts — for the vessel, which was attempting to dive about 400 miles off the coast of Newfoundland, Canada.

After the noises were detected, rescuers relocated two remotely operated vehicles (ROV) that search under the water and one surface vessel with sonar capability, the US Coast Guard said.

The ROV searches have so far not yielded results but data from the Canadian aircraft has been shared with US Navy experts for acoustics analysis.

“There have been multiple reports of noises and every one of those noises is being analysed,” said Carl Hartsfield, from the Woods Hole Oceanographic Institution.

He added that the sounds “have been described as banging noises.”

Frederick said the number of surface vessels in the search would double from five to 10 in the next 24 to 48 hours.

The Navy has sent a specialised winch system for lifting heavy objects from extreme depths, other equipment and personnel; and the Pentagon is deploying a third C-130 aircraft and three C-17s.

A deep-sea underwater robot sent by France’s oceanographic institute is expected to arrive in the area early Wednesday afternoon.

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

Rescuers estimate that passengers may run out of air on Thursday morning, based on the sub’s capacity to hold up to 96 hours of emergency air.

The mission was expected to be the only manned trip to the Titanic this year due to bad weather, Harding wrote in an Instagram post beforehand.

The Titanic hit an iceberg and sank in 1912 during its maiden voyage from England to New York with 2,224 passengers and crew on board. More than 1,500 people died.

It was found in 1985 and remains a lure for nautical experts and underwater tourists.

The pressure at that depth as measured in atmospheres is 400 times what it is at sea level.

Mike Reiss, an American television writer who visited the Titanic wreck on the same sub last year, told the BBC the experience was disorientating and that he had to sign a waiver that “mentions death three different times on page one.”

Alistair Greig, professor of marine engineering at University College London, has suggested two possible scenarios based on images of the Titan.

He said if it had an electrical or communications problem, it could have surfaced and remained floating, “waiting to be found” — bearing in mind the vessel can reportedly be unlocked from the outside only.

“Another scenario is the pressure hull was compromised — a leak,” he said in a statement.

“Then the prognosis is not good.”

In 2018, OceanGate Expeditions’ former director of marine operations David Lochridge alleged in a lawsuit that he had been fired after raising concerns about the company’s “experimental and untested design” of the craft.