United States: A "debris field" was discovered on Thursday by a robotic deep-sea vessel searching for a missing tourist submersible on the ocean floor near the wreck of the Titanic, the US Coast Guard said.

Experts are reviewing the information gathered by the remotely operated vehicle that was deployed from a Canadian ship, the agency said on Twitter, without offering additional details. A press conference to explain the findings was scheduled for 3 pm ET (1900 GMT).

The desperate search for the 22-foot (6.7-meter) Titan submersible had reached a critical stage on Thursday morning when the air supply for the five people on board was estimated to have nearly run out - or possibly run out.

The van-sized Titan, operated by US-based OceanGate Expeditions, began what was to be a two-hour descent at 8 am (1200 GMT) on Sunday but lost contact with its support ship.

The submersible set off with 96 hours of air, according to the company, which means the oxygen would likely have been exhausted by Thursday morning, assuming the Titan is still intact. Precisely when depends on factors such as whether the craft still has power and how calm those on board are, experts say.

Another robot from a French research ship also dove toward the seabed on Thursday to search for signs of the Titan submersible.

Even if located in time, a rescue operation would face enormous logistical challenges in retrieving the submersible from more than 2 miles below the surface.

Rescuers and relatives of the Titan's five occupants took hope when the US Coast Guard said on Wednesday that Canadian search planes had recorded undersea noises using sonar buoys earlier that day and on Tuesday.

But remote-controlled underwater vehicles searching where the noises were detected did not yield results, and officials cautioned the sounds might not have originated from the Titan.

US Coast Guard rear admiral John Mauger told broadcaster NBC earlier on Thursday that the search would continue throughout the day.

Deep-sea adventure

The Titanic, which sank in 1912 on its maiden voyage after hitting an iceberg, killing more than 1,500 people, lies about 900 miles (1,450 km) east of Cape Cod, Massachusetts, and 400 miles (640 km) south of St. John's, Newfoundland.

The Titan's deep-sea excursion to the shipwreck capped a tourist adventure for which OceanGate charges $250,000 per person.

The passengers included British billionaire and adventurer Hamish Harding, 58, and Pakistani-born business magnate Shahzada Dawood, 48, with his 19-year-old son Suleman, who are both British citizens.

French oceanographer and leading Titanic expert Paul-Henri Nargeolet, 77, and Stockton Rush, the US founder and chief executive of OceanGate, were also on board. Rush is married to a descendant of two of the Titanic victims.

"We're waiting anxiously, we hardly sleep," said Mathieu Johann, Nargeolet's editor at his publisher Harper Collins.

Questions about Titan's safety were raised in 2018 during a symposium of submersible industry experts and in a lawsuit filed by OceanGate's former head of marine operations, which was settled later that year.

If Titan were found intact on the ocean floor, a rescue would have to contend with the immense pressures and total darkness at that depth. British Titanic expert Tim Maltin said it would be "almost impossible to effect a sub-to-sub rescue" on the seabed.

It may also be difficult to find the Titan amid the wreck.

"If you've seen the Titanic debris field, there'll be a thousand different objects that size," said Jamie Pringle, a forensic geoscientist at Keele University in the United Kingdom. "It might be an endless task."