A submersible taking wealthy tourists to visit the site of the Titanic wreckage in deep waters off the coast of Canada was missing for a third day on Tuesday, as US and Canadian ships and planes swept a huge area trying to find the vessel.

One pilot and four passengers were on board the submersible that went missing on Sunday, the US 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 Suleman.

The 77-year-old French explorer Paul-Henri Nargeolet and Stockton Rush, founder and CEO of the vessel's US-based operating company OceanGate, were also reported to be 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.

US 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), US 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 US 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 to the site of the Titanic's wreckage on Sunday.

Billionaire aboard

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

Harding, a UAE-based businessman and adventurer who is chairman of Action Aviation, posted a message on his Facebook page on Saturday, saying: "Due to the worst winter in Newfoundland in 40 years, this mission is likely to be the first and only manned mission to the Titanic in 2023.

"A weather window has just opened up and we are going to attempt a dive tomorrow." He added: "More expedition updates to follow IF the weather holds!"

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

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 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 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".