'A catastrophic implosion.' The Titanic submarine passengers have been confirmed dead.

The five people aboard the missing submersible travelling to the Titanic have died in what appears to have been a "catastrophic implosion," a US coast guard official says.

Days into a massive international search, an unmanned deep-sea robot deployed from a Canadian ship discovered the wreckage of the Titan on Thursday morning about 488 metres from the bow of the century-old Titanic ship wreck and 4km below the surface. 

"The debris field here is consistent with a catastrophic implosion of the vehicle," US coast guard Rear Admiral John Mauger said at a press conference.

Debris has been found that is “consistent with the catastrophic loss of the pressure chamber” of the missing Titan submersible, U.S. Coast Guard Rear Admiral John Mauger says.

The Coast Guard notified the families that all five people on board are believed to be dead. pic.twitter.com/dGLXAGsB2H

— The Recount (@therecount) June 22, 2023

The five people aboard included 58-year-old British billionaire and explorer Hamish Harding, 48-year-old Pakistani-born business magnate Shahzada Dawood, and his 19-year-old son, Suleman, 77-year-old French oceanographer and Titanic expert Paul-Henri Nargeolet and 61-year-old Stockton Rush, the US founder and chief executive of OceanGate, who was piloting the submersible.

"These men were true explorers who shared a distinct spirit of adventure, and a deep passion for exploring and protecting the world's oceans," OceanGate Expeditions, who operated the submersible, said in a statement.

"Our hearts are with these five souls and every member of their families during this tragic time."

Rescue teams from several countries had spent days searching thousands of square miles of open seas with planes and ships for any sign of the 6.7-metre Titan.

The submersible lost contact on Sunday morning with its support ship about an hour and 45 minutes into what should have been a two-hour descent.

Mauger said it was too early to tell whether the vessel's failure occurred then or at a later time.

The detection of undersea noises on Tuesday and Wednesday using sonar buoys dropped from Canadian aircraft had temporarily offered hope that the people on board the submersible were alive and trying to communicate by banging on the hull.

But officials had warned that analysis of the sound was inconclusive and that the noises might not have emanated from the Titan at all.

"There doesn't appear to be any relation between the noises and the location of the debris field on the sea floor," Mauger said on Thursday.

The search had grown increasingly desperate on Thursday, when the estimated 96-hour air supply was expected to run out if the Titan were still intact.

The Titanic, which sank in 1912 on its maiden voyage after hitting an iceberg, killing more than 1500 people, lies about 1450km east of Cape Cod, Massachusetts. 

The expedition to the wreck, which OceanGate has been operating since 2021, cost $US250,000 ($A368,000) per person, according to OceanGate's website.

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

Read more: 

'Banging' noises and less than 24 hours left: How the search for the missing submarine is unfolding.

- With AAP. 

Feature Image: AAP/Engro Corporation Limited/OceanGate/Facebook/Getty/Twitter.