The Titan OceanGate Expeditions submersible is believed to have suffered a "catastrophic implosion" soon after it went missing, the US Navy has said.

The five people on board, who were chief executive of the company Stockton Rush, British citizens Hamish Harding, Shahzada Dawood and his 19-year-old son Suleman Dawood and French national Paul-Henri Nargeolet, are believed to be dead.

In a statement, OceanGate Expeditions said: “These men were true explorers who shared a distinct spirit of adventure, and a deep passion for exploring and protecting the world’s oceans.

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

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

The US Coast Guard offered its “deepest condolences” to the families after the tail cone of the submersible was found around 1,600ft from the bow of the Titanic wreckage.

In a press conference, Rear Admiral John Mauger said further debris was “consistent with a catastrophic loss of the pressure chamber”.

Debris from Titanic tourist submarine discovered

The discovery of debris from the submersible comes as a senior official in the navy confirmed that an existing secret system designed to pick up audio heard a sound consistent with an "implosion".

The sound was heard near where the submersible lost communication on Sunday (June 18), Sky News reported.

While the official said the sound was "not definitive", the navy said it shared the information "immediately" with the incident commander.

A navy statement said: "This information was considered with the compilation of additional acoustic data provided by other partners and the decision was made to continue our mission as a search and rescue and make every effort to save the lives on board."

When asked about the prospects for recovering crew members, Rear Admiral Mauger warned "it is an incredibly unforgiving environment on the seafloor".

While the debris is consistent with a "catastrophic implosion" of the vessel, he stressed that it is too early to know when this happened - and underwater robots remain on scene to gather information.