The Titan, which went missing Sunday, has a 96-hour oxygen supply in case of emergency

The OceanGate-owned vessel, with five people on board, lost communication with tour operators less than two hours into its dive on Sunday while about 435 miles south of St John's, Newfoundland during a voyage to the Titanic shipwreck off the coast of Canada.

British billionaire Hamish Harding, OceanGate CEO Stockton Rush, French navy veteran PH Nargeolet and Pakistani businessman Shahzada Dawood and his son Suleman are onboard the vessel.

As a massive international rescue effort continues, a grim countdown has also begun with the vital oxygen supply expected to run out at 7.08am US eastern time, 12.08pm GMT, a US Coast Guard spokeswoman has said.

However, rescuers have stressed it is "100 per cent" still a search and rescue mission, and not a salvage exercise.

US Coastguard Captain Jamie Frederick insisted on Wednesday during a press conference that there was still hope of a successful rescue, saying: "We have to remain optimistic and hopeful when we are in a search and rescue case."

"If we continue to search, potentially we could be at that point… And that's a discussion we will have with the families long before I am going to discuss here publicly."

An expert earlier this week predicted the chances of the group being rescued safely was one per cent.

OceanGate has said the submersible has a 96-hour oxygen supply in case of emergencies.

Search and rescue teams are scouring an area where noises were detected for the missing sub, with experts saying they are continuing to analyse the sounds which were heard as recently as this morning.

The only possible trace of the vessel which is continuing to be investigated was underwater "banging" sounds, which were detected Wednesday, but, the US Navy experts analysing the sounds said they cannot conclude whether they are coming from the stranded vessel.

More specialised equipment was due to arrive Thursday morning to join the search.

The search area is reportedly been expanded to 14,000 square miles.

The trip, which is thought to cost £195,000 per head, launched at 4am Sunday. Communications disappeared one hour and 45 minutes into the descent to the wreck site – which sits about 3,800m (12,500ft) below sea level at the bottom of the ocean around 370 miles off the coast of Newfoundland but in US waters.

The expedition was OceanGate’s third annual voyage to chronicle the deterioration of the iconic ocean liner that struck an iceberg and sank in 1912.

Related links:

Loose Women star Janet Street-Porter calls missing Titanic explorers ‘selfish billionaires’

Submarine expert gives grim prediction on survival chances of those on board

‘Claustrophobic’ photos show how small the submersible is that went missing during Titanic tour

Submarine that went missing on tour of Titanic will ‘run out of oxygen in 48 hours’

British billionaire among five still missing on Titanic submarine