BOSTON – A multinational mission to find a missing submersible near the Titanic wreck is still focused on rescuing the five-member crew alive, the US Coast Guard insisted on Thursday, despite fears that the vessels oxygen may already have run out.

Based on the submersibles capacity to hold up to 96 hours of emergency air, rescuers had estimated that the passengers could run out of oxygen in the early hours of Thursday.

But as that possible deadline passed, US Coast Guard Rear-Admiral John Mauger said rescuers were fully committed.

Peoples will to live really needs to be accounted for as well. And so were continuing to search and proceed with rescue efforts, he told NBCs Today show.

A remotely operated vehicle deployed from a Canadian vessel reached the ocean floor to begin searching, the US Coast Guard said on Thursday morning, while another robotic craft from a French research ship was also preparing to dive to the seabed.

The minivan-sized Titan, operated by US-based OceanGate Expeditions, began its descent at 8am on Sunday but lost contact with its support ship.

Despite the fading hopes, Rear-Admiral Mauger told broadcaster NBC the search would continue throughout Thursday.

The French research ship Atalante, equipped with a robotic diving craft capable of reaching where the Titanic lies about 3,810m below the surface, had arrived in the zone as of Thursday.

It was first using an echo-sounder to accurately map the seabed for the robots search to be more targeted, the French marine research institute Ifremer said.

The robot, Victor 6000, has arms that can be remotely controlled to help free a trapped craft or hook it to a ship to haul it up. The US Navy is sending a special salvage system designed to lift large undersea objects.

A surge of assets and experts have joined the operation, and sonar has picked up unidentified underwater noises.

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 the underwater noises detected by sonar. Remote video URL Mr Mauger has said that vessels carrying medical staff and a decompression chamber are en route to the area.

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

We dont know what they are, to be frank with you, said US Coast Guard Captain Jamie Frederick.

We have to remain optimistic and hopeful.

Titan was carrying British billionaire Hamish Harding and Pakistani tycoon Shahzada Dawood and his son Suleman, who also have British citizenship.

OceanGate Expeditions charges US$250,000 (S$335,000) for a seat on the sub. Mr Titanic

Also on board is the companys chief executive, Mr Stockton Rush, and a French submarine operator Paul-Henri Nargeolet, nicknamed Mr Titanic for his frequent dives at the site. Titan was carrying (clockwise from top left) British billionaire Hamish Harding, OceanGate Expeditions CEO Stockton Rush, Pakistani tycoon Shahzada Dawood and his son Suleman, and French submarine operator Paul-Henri Nargeolet. PHOTO: AFP Ships and planes have scoured around 20,000 sq km of surface water roughly the size of the US state of Massachusetts for the vessel, which attempted to dive about 644km off the coast of Newfoundland, Canada.

After the noises were detected by a Canadian P-3 aircraft, rescuers relocated two remotely operated vehicles (ROV) that search under the water and one surface vessel with sonar capability.

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

The Navy has sent a specialised winch system for lifting heavy objects from extreme depths along with other equipment and personnel, while the Pentagon has deployed three C-130 aircraft and three C-17s. Remote video URL 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.

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 companys experimental and untested design of the craft. More On This Topic OceanGate was warned of potential for catastrophic problems with Titanic mission Missing Titanic tourist vessel: Best and worst-case scenarios Inside the sub

Mr Tom Zaller, who runs the company behind Titanic: The Exhibition, toured the wreck 23 years ago in a submersible much like the one that went missing on Sunday.

Youre sending a very small vessel two and a half miles down, which is incredibly complicated and technical, he said.

Its just this very seemingly unsophisticated sphere.

Mr Zaller has known Mr Nargeolet for decades and was in touch with Mr Rush before he embarked on Sundays tour.

I was in that sub for 12 hours with everything working fine, Mr Zaller said. Theyve been there for almost four days. I just cant imagine. More On This Topic Inside the Titan submersible: Quiet and cramped, with a single porthole Missing Titanic tourist vessel: Difference between a submersible and a submarine Deep-sea adventure

Mr Sean Leet, who heads a company that jointly owns the support ship, the Polar Prince, has said all protocols were followed before the submersible lost contact.

Theres still life support available on the submersible and well continue to hold out hope until the very end, said Mr Leet, chief executive of Miawpukek Horizon Maritime Services.

Questions about Titans safety were raised in 2018 during a symposium of submersible industry experts and in a lawsuit filed by OceanGates former head of marine operations, which was settled later that year. Remote video URL Even if the Titan were located, retrieving it would present huge logistical challenges.

If the submersible had managed to return to the surface, spotting it would be difficult in the open sea and it is bolted shut from the outside, so those inside cannot exit without help.

If Titan is on the ocean floor, a rescue would have to contend with the immense pressures and total darkness at that depth. British Titanic expert Tim Maltin said it would be almost impossible to effect a sub-to-sub rescue on the seabed.

It may also be difficult to find the Titan amid the wreck.

If youve seen the Titanic debris field, therell be a thousand different objects that size, said Dr Jamie Pringle, a forensic geoscientist at Keele University in the United Kingdom. It might be an endless task. AFP, REUTERS More On This Topic Cold and dark: The journey to the bottom of the sea on a Titanic tour TheTitanic: The enduring allure of one of the worlds deadliest ocean disasters