Updated 9.30pm

The operator of a submersible missing near the wreck of the Titanic said Thursday it believed all five people on board were dead, due to what the Coast Guard described as a "catastrophic" implosion of the vessel in the ocean depths.

The solemn announcement from Oceangate came hours after rescuers said a "debris field" had been discovered following a days long search - adding to fears that, even if the vessel were still intact, its oxygen may have run out.

"We now believe that our CEO Stockton Rush, (Maltese citizen) Shahzada Dawood and his son Suleman Dawood, Hamish Harding, and Paul-Henri Nargeolet, have sadly been lost," Oceangate said in a statement.

"These men were true explorers who shared a distinct spirit of adventure, and a deep passion for exploring and protecting the world's oceans," it said.

"We grieve the loss of life and joy they brought to everyone they knew."

The US Coast Guard said the debris was consistent with a "catastrophic implosion" of the vessel.

The small tourist sub had been lost since Sunday, somewhere in a vast swathe of the North Atlantic between the ocean's surface and more than two miles (nearly four kilometers) below.

It was carrying British billionaire Harding and dual Pakistani-British citizens Dawood, a tycoon, and his son Suleman. OceanGate Expeditions charges $250,000 for a seat on the sub.

Also on board was OceanGate's CEO Rush and French submarine operator Nargeolet, nicknamed "Mr Titanic" for his frequent dives at the site.

Marine scientist and oceanographer David Mearns, who specializes in deep water search and recovery operations, earlier said the discovery of debris indicated a breakup of the submersible.

"The only saving grace about that is that it would have been immediate, literally in milliseconds, and the men would have had no idea what was happening," Mearns, who was friends with two of those onboard, told Sky News.

Surge of assets

A surge of assets and experts had joined the operation in the past day, including two more Remotely Operated Vehicles (ROVs).

The French research ship Atalante deployed an unmanned robot able to search at depths of up to 6,000 metrers below water on Thursday, the US Coast Guard tweeted.

The 6.5-metre Titan began its descent at 8 am on Sunday and had been due to resurface seven hours later.

But the craft lost communication with its mothership less than two hours into its trip to see the Titanic.

Ships and planes scoured around 20,000 square kilometres of surface water - roughly the size of the US state of Massachusetts - for the vessel.

The Titanic's watery grave is situated 400 miles off the coast of Newfoundland, Canada, and more than two miles below the surface of the North Atlantic.

The Titanic hit an iceberg and sank in 1912 during its maiden voyage from England to New York with 2,224 passengers and crew on board. More than 1,500 people died.

It was found in 1985 and remains a lure for nautical experts and underwater tourists.

The pressure at that depth as measured in atmospheres is 400 times what it is at sea level.

In 2018, OceanGate Expeditions' former director of marine operations David Lochridge alleged in a lawsuit that he had been fired after raising concerns about the company's "experimental and untested design" of Titan.