The US Coast Guard and other search and rescue crews have swarmed a remote area in the Atlantic in search for the vessel, called Titan, which went missing on Sunday with five people on board.

Coast Guard Rear Admiral John Mauger told CNN: "As we continue on with this search ... we've been working through the night with a broad group of partners to bring all capabilities to bear looking on both the surface and now expanding to a subsurface in the area."

NATO rescue vessel cannot reach required depths - missing Titanic sub latest

Rescuers face a race against time to find the missing submersible as its oxygen supply dwindles.

It is understood from the vessel's operator, OceanGate Expeditions, that Titan had a 96-hour oxygen supply in case of emergencies - meaning the people on board have around two days of "life support" left.

The five people on board are British billionaire Hamish Harding, Pakistani businessman Shahzada Dawood and his son Suleman, chief executive and founder of OceanGate Expeditions Stockton Rush, and French submersible pilot Paul-Henri Nargeolet.

Mr Dawood is married and has two children, the family said in a statement. He is vice chairman of Pakistan's Engro Corporation.

He and his son Suleman, who is currently studying at university, are British citizens.

Read more:
What we know about missing Titanic sub
Why the hunt for the missing vessel is so difficult

A major search and rescue operation is taking place some 435 miles south of Newfoundland, Canada, involving the US Coast Guard as well as Canadian and US aircraft.

The Canadian research icebreaker Polar Prince, which was supporting the Titan, reportedly lost contact with the vessel about an hour and 45 minutes after it submerged.

The wreckage of the Titanic that sank in 1912 after hitting an iceberg lies at about 12,500 feet (3,810 metres).

The Titan submersible usually takes two hours to descend to the wreck.

'Terrified if they are stuck at bottom of ocean'

Jannicke Mikkelsen, an explorer and friend of Hamish Harding, told Sky News that she was "terrified" if the submersible was stuck at the bottom of the sea.

However, she said that she believed Mr Harding would be an "important asset" to the others in the submersible.

"Hamish will definitely be an important asset to the team, and I have no doubt he will be calm and collected," she said.

"And he will go through the emergency checklists and procedures together with the captain of the submersible.

"He will also be a good inspiration to the rest of the team to keep calm. I really believe Hamish is the one that can help lead this team - because he has been in tricky situations in the past on expeditions.

She added: "I'm terrified if they are stuck at the bottom of the ocean with 96 hours of air and not able to get back to the surface - this is what I fear the most."

Could the Titan have moved close to the surface?

But it was still unclear whether the vessel had resurfaced - or how close to the surface it might have got.

It could have got entangled in the Titanic wreak, it could have sunk deeper, or it could float eventually to the surface.

If the sub got into trouble and warning alerts were triggered, emergency measures should have been implemented to bring the submersible to the surface, Sky News understands.

These would have included dropping weights to bring the sub slowly to the surface.

It is unknown if there was an automatic system on board or when systems would automatically trigger the release.

Additionally measures including adding air to ballast tanks would also have helped bring the submersible to the surface.

In a worst-case scenario, if the sub's pressure hull imploded, there would have been no survivors.