That is despite US Coast Guard estimates that the air supply in the vessel would run out at 12.08pm UK time on Thursday.

The vessel - operated by the US-based firm OceanGate - had five men on board when it went missing: British billionaire Hamish Harding, British businessman Shahzada Dawood, his son Suleman Dawood, OceanGate's US-based chief executive and founder Stockton Rush and French submersible pilot Paul-Henri Nargeolet.

Rear Admiral John Mauger, from the US Coast Guard, told Sky News rescuers "remain hopeful" the submersible can be found, but revealed the "banging noises" detected by the sonar buoys dropped into the sea were possibly "ocean background noise".

Titanic sub search - live: Rescue robot reaches sea floor - as oxygen forecast to have run out

He said: "We continue to keep the crew members and families in our thoughts.

"While we are cognisant of the time and we've factored in a lot of data and information into this search, this is still an active search and rescue at this point.

"We are using the equipment that we have on the bottom right now, remote-operated vehicles to expand our search capability, and also to provide rescue capability."

He said conditions for the operation were "favourable" - and crews were making the most of the "weather window and the good conditions".

Aircraft continue to fly over the area looking for the submersible on the surface, he said, but the "focus right now is on that sub-sea search", with the remotely-operated vehicles scouring the pathline where the planned dive had taken place.

The vessel disappeared on Sunday in the Atlantic Ocean, some 435 miles south of Newfoundland, Canada, during an expedition dive to see the wreck of the Titanic.

It lost communication with tour operators about an hour and 45 minutes after the vessel submerged.

Ships, planes and underwater craft from multiple countries have since been deployed to the area with rescuers advised by world-leading experts searching a remote part of the Atlantic Ocean more than twice the size of the US state of Connecticut in waters as deep as 4,020m (13,200ft).

Two ROVs searching for sub on sea floor

One of the vessels sent to help search efforts is French research vessel L'Atalante which has deployed a remotely operated vehicle (ROV), Victor 6000.

The unmanned French robot, which can dive to a depth of 6,000 metres (19,600ft), reached the search site a few hours before the critical 96-hour mark when oxygen was due to run out.

The wreck of the Titanic, which the Titan was heading towards, lies at a depth of about 3,810 metres (12,500ft).

While the vehicle cannot lift Titan to the surface on its own, it could help to hook it to a ship on the surface.

A second ROV - deployed by Canadian vessel Horizon Arctic - has also reached the sea floor and is looking for the missing submersible.

Both ROVs have cameras and sonar.

Mr Mauger said it was an "incredibly complex effort to get this equipment on site", adding: "They have articulated arms that are capable of attaching equipment and attaching a rescue line."

When will the rescue effort end?

Reports of underwater sounds in the area are still being investigated but it is believed much of it may be ocean background noise.

Asked about the sounds, Mr Mauger said that analysis of these by experts is ongoing.

"The initial report is that there's a lot of the sounds that were generated were from background ocean noise, but they continue to... look for all available information there," he said.

"What's important to me... is that we've continued to search in the areas where noise was detected with the ROVs that we had from the time of that detection, so we're not waiting for this analysis to take action."

He said any decision to stop the rescue effort would be taken after a "thorough consultation" with the families of the passengers on the submersible.

"There's a time and place for that discussion. Right now... we will remain focused on the search and rescue but continue to take in all available information as we plan out our future operation."

He said there would in due course be an investigation into what has led to the submersible going missing.

"There's going to be a time and place for a thorough investigation," he added.

'Sub's life support supplies are starting to run low'

In a statement published online Guillermo Sohnlein, co-founder of OceanGate, thanked the people involved in the search and rescue - and claimed the time window the people on board have is larger than the estimated 96 hours.

"Today will be a critical day in this search and rescue mission, as the sub's life support supplies are starting to run low," he said in a statement.

"I'm certain that Stockton and the rest of the crew realised days ago that the best thing they can do to ensure their rescue is to extend the limits of those supplies by relaxing as much as possible.

"I firmly believe that the time window available for their rescue is longer than what most people think.

"I would encourage everyone to remain hopeful for getting the crew back safely."

However, some experts have said the chances of finding the sub and rescuing those inside is diminishing.

The situation is "gloomy" and "we have to prepare ourselves for the worst", retired rear admiral Chris Parry told Sky News.

He said the chances of survival for the Titan crew are "vanishingly small", given the "extreme limits for their oxygen".