A frantic search-and-rescue mission is in a race against time to find a missing tourism submersible – with five people on board – that has limited oxygen, food and water.

Authorities said the Titan sub, which was exploring the Titanic wreck when it went missing on Sunday, might have had 70 to 96 hours of oxygen available when it lost contact.

A complex operation is underway in a remote area of the Atlantic Ocean involving US and Canadian ships and planes.

The US Coast Guard said it was unclear whether the Titan was still underwater or had surfaced and was unable to communicate.

One pilot and four passengers were on board, including British billionaire Hamish Harding, when it was reported missing less than two hours after beginning its dive.

The Titanic wreck lies nearly four kilometres underwater in the great depths of the Atlantic Ocean. It takes about two hours for the OceanGate submarine to make the long drop to the floor for the rare and risky experience of getting close to the famous sunken ship.

CBS correspondent David Pogue, who took a dive trip last year, told the ABC it was likely the Titan would by now have only about half its supply of oxygen remaining.

How fast that oxygen was used up would depend on the degree of panic among the crew.

There was also limited food and water on board for the short dive trips, and no heating or cooling in the “really cold” space.

Pogue said the best-case scenario was that the Titan had reached the surface and would be spotted by its white colour against the blue ocean.

But the “scarier” alternative was that the craft and people were still deep down which raised the question of how they could be rescued.

Pogue said the crew could not exit the Titan that far underwater because they would be “crushed” by the extreme water pressure.

During his trip last year, the journalist reported it was the only five-person vessel of its kind in the world, and an unregulated and non-certified experimental sub.

Image: Getty

Rescuers had swarmed an area about 1450 kilometres east of Cape Cod, some dropping sonar buoys that can monitor to a depth of 3962 metres, said US Coast Guard Rear Admiral John Mauger.

“It is a remote area and it is a challenge to conduct a search in that remote area,” Mr Mauger said.

“We are deploying all available assets to make sure that we can locate the craft and rescue the people on board,” he said.

“Going into this evening we will continue to fly aircraft and move additional vessels.”

OceanGate said it was “exploring and mobilising all options to” find the missing sub and its five passengers.

Our entire focus is on the crew members in the submersible and their families,” the company said in a Facebook post late on Monday (local time).

“We are deeply thankful for the extensive assistance we have received from several government agencies and deep sea companies in our efforts to reestablish contact with the submersible.”

The expedition set off from Newfoundland by ship before reaching the site when passengers who had paid $US250,000 a ticket ($364,000) entered the Titan submersible.

Experts quoted in media reports have proffered several scenarios that may have gone wrong.

These include the possibility that the submersible had become “entangled” in the Titanic wreck, which is surrounded by debris and protrusions.

“There are parts of it all over the place. It’s dangerous,” Frank Owen OAM, a retired Royal Australian Navy official and submarine escape and rescue project director told The Guardian.

Other expert suggestions are that there may have been a power failure, problem with the communications system, or a leak in the pressure hull which would have a drastic outcome.

Contact with the pilot ship was lost an hour an 45 minutes into the Titan’s dive, which Mr Owen said indicated the crew may have been close to, or at, the bottom.

Royal Navy retired rear admiral Chris Parry told Sky News a seabed rescue was “a very difficult operation”.

“The actual nature of the seabed is very undulating. Titanic herself lies in a trench. There’s lots of debris around,” Mr Parry said.

“Trying to differentiate with sonar in particular and trying to target the area you want to search in with another submersible is going to be very difficult indeed.”