Rescue workers continue the hunt for the Titanic submersible with the oxygen supply on board expected to run out at 12.08pm UK time, a US Coast Guard spokesperson has said.
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 6.7m-long OceanGate Expeditions vessel, which has British billionaire adventurer Hamish Harding on board, reportedly had a 96-hour oxygen supply in case of emergencies.
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.
As of Wednesday afternoon, it was thought just 20 hours of oxygen remained in the vessel, meaning it would run out at some point on Thursday morning.
And a US coast guard spokeswoman has said it will run out at exactly 12.08pm today.
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."
One of the vessels sent to help search efforts is French research ship L'Atalante, which carries a remotely operated vehicle (ROV), Victor 6000.
This ROV has the capacity to lift the Titan ship to the surface.
Mr Marquet said he remains hopeful rescuers will find the vessel with the introduction of the ROVs.
The area of the search has been expanded, with the surface search now about 10,000 square miles, and the sub-surface search about 2.5 miles deep.
The coast guard 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.
He added: "What I can tell you is, we're searching in the area where the noises were detected, and we'll continue to do so. We hope that when we're able to get additional ROVs (remotely operated vehicles) which will be there in the morning, the intent will be to continue to search in those areas where the noises were detected, and if they're continuing to be detected, and then put additional ROVs down on the last known position where the search was originally taking place."
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."
Titan is believed to be about 900 miles east and 400 miles south of Newfoundland. It is not known how deep the vessel is, with the seabed being around 3,800m from the surface.
Questions have been raised about the vessel after it emerged earlier in the week that a former employee of OceanGate questioned "safety and quality control issues regarding the Titan to OceanGate executive management".
David Lochridge, OceanGate's former director of marine operations, claimed in an August 2018 court document that he was wrongfully fired after flagging worries about the company's alleged "refusal to conduct critical, non-destructive testing of the experimental design".
On Wednesday Kathleen Cosnett, a cousin of Mr Harding, 58, 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."
Sean Leet, co-founder and chairman of Horizon Maritime Services, which owns the Polar Prince mothership from which Titan launched, defended the company at a separate press conference on Wednesday.
He said: "OceanGate runs an extremely safe operation. Our full focus right now is getting that submersible located and getting those people brought back safely."
