Contact with the small sub was lost about an hour and 45 minutes into its dive, the US Coast Guard was quoted as saying by BBC.

Tour firm OceanGate said: "We are working toward the safe return of the crew members."

US Coast Guard also issued a statement on the development and said: "The Coast Guard is searching for five persons after the Canadian research vessel Polar Prince lost contact with their submersible during a dive, approximately 900 miles east of Cape Cod, Sunday morning."

"A Coast Guard Air Station Elizabeth City, North Carolina, C-130 Hercules aircraft, as well as a Canadian P8 aircraft equipped with underwater sonar capability, are currently searching for the missing submersible," read the statement.

Tickets cost $250,000 (£195,000) for an eight-day trip including dives to the wreck at a depth of 3,800m (12,500ft), reports BBC.

Government agencies, the US and Canadian navies and commercial deep-sea firms are reportedly helping the rescue operation.