One pilot and four passengers were on board the submersible that went missing on Sunday, the U.S. Coast Guard said, adding the vessel could stay underwater for up to 96 hours, although it was unclear if it had resurfaced but was unable to communicate.

Those aboard the submersible called Titan, the highlight of a tourist expedition that costs $250,000 per person, included British billionaire Hamish Harding and Pakistani businessman Shahzada Dawood with his son. French media also reported that 77-year-old French explorer Paul-Henri Nargeolet was on board.

"We are very grateful for the concern being shown by our colleagues and friends and would like to request everyone to pray for their safety," Dawood's family said on Tuesday.

U.S. and Canadian ships and planes began swarming the area on Monday about 900 miles (1,450 km) east of Cape Cod, some dropping sonar buoys that can monitor to a depth of 13,000 feet (3,962 metres), U.S. Coast Guard Rear Admiral John Mauger said.

"We are deploying all available assets to make sure that we can locate the craft and rescue the people on board," he told reporters on Monday. "It is a remote area and it is a challenge to conduct a search in that remote area."

He said officials had asked commercial vessels to help.

The wreckage of the Titanic that sank in 1912 after hitting an iceberg lies at about 12,500 feet (3,810 metres). The Titan submersible usually takes two hours to descend to the wreck.

OceanGate Expeditions, the private company that operates the submersible, said it was "mobilizing all options" to rescue those aboard the Titan.

The U.S. Coast Guard said on Twitter that a boat on the surface - the Polar Prince - lost contact with the submersible about one hour and 45 minutes after it began diving toward to the site of the Titanic's wreckage on Sunday.

Harding's stepson wrote on Facebook that Harding had "gone missing on submarine" and asked for "thoughts and prayers", but later removed the post citing respect for the family's privacy.

BILLIONAIRE ABOARD

Harding, a UAE-based businessman and adventurer who is chairman of Action Aviation, had posted on Facebook that he would be aboard the submersible.

The expedition headed out to sea on Friday, and the first dive was set for Sunday morning, according to Harding's post.

Fellow tourist, Dawood, is the vice chairman of one of Engro Corporation, one of Pakistan's largest conglomerates with investments ranging from fertilisers and energy to vehicle manufacturing.

SETI, the California-based research institute of which he is a trustee, said on its website that Dawood lives in Britain with his wife and two children.

The expeditions start in St. John's, Newfoundland, before heading out approximately 400 miles (640 km) into the Atlantic to the wreckage site, according to OceanGate's website.

The British passenger ship sank on its maiden voyage, killing more than 1,500 people, a tragedy that has been immortalized in books and films, including the 1997 blockbuster movie "Titanic."

(Reporting by Joseph Ax and Kanishka Singh in Washington, Ismail Shakil and Ariba Shahid in Karachi; Editing by Edmund Blair and Janet Lawrence)

By Joseph Ax and Kanishka Singh