Debris discovered on the ocean floor suggests the missing submersible near the wreck of the Titanic suffered a "catastrophic loss" of pressure, the US Coast Guard said Thursday.

"Upon this determination, we immediately notified the families," Rear Admiral John Mauger told reporters in Boston.

"On behalf of the United States Coast Guard and the entire unified command, I offer my deepest condolences to the families."

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The 21-foot (6.5-meter) craft, operated by OceanGate Expeditions, began its descent on Sunday but lost contact with the surface less than two hours later, according to authorities.