A US Navy salvage system has arrived in Newfoundland as oxygen on board the missing Titan sub dwindles into its final hours with no sign of the vessel.

A Navy official said crews in the Canadian town of St Johns were preparing to send the salvage system to the vast search zone in the North Atlantic.

It came as a former US Navy nuclear submarine commander David Marquet warned that hopeful sounds detected repeatedly in recent hours might not be coming from the Titan submersible.

“I don’t think the noise is them, it could just be natural sounds. We’re hearing noises and more ships are coming into the area, and then we’re hearing more noises, and I don’t think that’s a coincidence,” he told the BBC late on Wednesday.

The US Coast Guard has projected the missing sub’s oxygen supply will expire at 7.08am Thursday ET (9.08pm  AEST).

The Coast Guard is using remotely operated vehicles to search underwater near where Canadian aircraft recorded the noises earlier this week. They are yet to find any trace of the Titan or its occupants.

Coast Guard Captain Jamie Frederick said analysis of the noises had been “inconclusive”.

“When you’re in the middle of a search-and-rescue case, you always have hope,” he said.

“With respect to the noises specifically, we don’t know what they are, to be frank with you.”

Captain Marquet said the odds of survival for the five passengers on board the Titan were low, but had risen slight with the salvage equipment’s arrival.

“It’ll be desperately close because it needs to be found before then,” he said.

The Flyaway Deep Ocean Salvage System (FADOSS) can retrieve objects or vessels off the bottom of the ocean floor up to 6000 metres deep – more than enough to reach the wrecked Titanic, which is at 3800 metres.

But before the system can be used, it must be welded to the deck of a ship. An official said that process could take a full day.

“Our estimate is approximately 24 hours of round-the-clock operation to weld it and secure it to the deck of the vessel prior to getting underway,” the official said.

Former US Navy diver Bobbie Scholley said there was still cause for hope, despite initial reports oxygen on board the Titan would run out by early Thursday.

“Everybody is focused on the 96-hour window for the life support that they’re giving to the crew but that’s not a hard and fast number and I know that the search teams are not focused on that being a hard and fast number,” Captain Scholley told the BBC.

“This will continue to be a rescue mission even past that number, if it has to go past that number. So that gives me hope also.”

Rescuers searching for the missing submersible are concentrating their efforts on a remote area of the North Atlantic where the undersea noises have been detected.

An international coalition of rescue teams has swept a vast expanse of the ocean for the Titan, which disappeared on Sunday while taking five people deep underwater to visit the century-old wreck.

Even if the submersible is located, retrieving it presents huge logistical challenges given the extreme conditions thousands of metres below the surface.

Teams from the US, Canada and France have used planes and ships to search more than 25,900 square kilometres of open sea, roughly twice the size of the US state of Connecticut.

The 6.7-metre submersible Titan, operated by US-based OceanGate Expeditions, began its descent at 8am on Sunday. It lost contact with its surface support ship during what should have been a two-hour dive to the Titanic.

Sean Leet, who heads a company that jointly owns the support ship, said “all protocols were followed” but declined to give a detailed account of how communication ceased.

“There’s still life support available on the submersible and we’ll continue to hold out hope until the very end,” Mr Leet said.

Experts said the duration of the air supply on board depended on a range of factors, including whether the Titan still had power and how calm its passengers had remained.

Those aboard the submersible, the highlight of a tourist adventure that costs $US250,000 ($A367,000) per person, included British billionaire and adventurer Hamish Harding, 58, and Pakistani-born businessman Shahzada Dawood, 48, with his 19-year-old son Suleman, who are both British citizens.

French explorer Paul-Henri Nargeolet, 77, and Stockton Rush, founder and chief executive of OceanGate Expeditions, were also reported to be on board.

Aaron Newman, a former Titan passenger who knows some of the missing people, told NBC on Wednesday he felt safe during his dive.

“This is not a Disney ride,” he said.

“We’re going places that very few people have been.”

If the Titan managed to return to the surface, it could still be difficult to spot it in the open water, experts said.

The submersible is sealed shut with bolts from the outside, making it impossible for those inside to escape without assistance.

If the Titan is on the ocean floor, a rescue effort would be even more challenging because of the massive pressures and total darkness at a depth of more than several kilometres.

Titanic expert Tim Maltin said it would be “almost impossible to effect a sub-to-sub rescue” on the seabed.

-with AAP