Boston, Massachusetts - Oxygen on the missing Titanic submersible is expected to run out in hours, with rescue efforts to find the five people onboard in full force overnight.

The vessel, named Titan, lost communication with tour operators on Sunday while about 435 miles south of St John’s, Newfoundland, during a voyage to the Titanic shipwreck off the coast of Canada.

The oxygen supply on the vessel is expected to run out at some point on Thursday morning.

The 22-foot-long OceanGate Expeditions vessel, which has British billionaire adventurer Hamish Harding on board, reportedly had a 96-hour oxygen supply in case of emergencies.

Mystery Sonar image revealed to be rock formation, not Amelia Earhart plane

Also in the undersea craft are UK-based businessman Shahzada Dawood, his son Suleman Dawood, and OceanGate’s chief executive and founder Stockton Rush, reportedly with French submersible pilot Paul-Henri Nargeolet.

The US Coast Guard has been leading an international rescue effort which was stepped up after underwater noises were heard on Tuesday and again on Wednesday, although experts have been unable to determine the cause of the sound.

Former US Navy submarine commander, David Marquet, told the BBC the noises may not be coming from the submersible.

"I don't think the noise is them, it could just be natural sounds," he said.

"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."

The coastguard had five surface vessels searching for Titan on Wednesday and they expected there to be 10 by Thursday, captain Jamie Frederick said at a press conference on Wednesday.

Asked whether the mission was changing to become a recovery search, he said: "This is a search and rescue mission 100%, we are smack dab in the middle of search and rescue and will continue to put every available asset that we have in an effort to find the Titan and the crew members."

Questions have been raised about the safety of the submersible after it emerged earlier in the week that a former employee of OceanGate had raised concerns over "safety and quality control issues regarding the Titan to OceanGate executive management."

Mystery Celebrated filmmaker disappears from New York beach without a trace

On Wednesday Kathleen Cosnett, a cousin of Harding, told the Telegraph that OceanGate's eight-hour delay before contacting the authorities was "far too long."

She said: "It's very frightening. It took so long for them to get going to rescue them, it's far too long. I would have thought three hours would be the bare minimum."