The US Coast Guard said there was one pilot and four passengers on board and that the vessel had the capacity to be submerged for 96 hours, but it was unclear whether it was still underwater or had surfaced and was unable to communicate.

US and Canadian ships and planes have swarmed the area 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 meters), US Coast Guard Rear Admiral John Mauger told reporters on Monday.

"It is a remote area and it is a challenge to conduct a search in that remote area," Mauger said. 

"We are deploying all available assets to make sure that we can locate the craft and rescue the people on board," he said. "Going into this evening we will continue to fly aircraft and move additional vessels."

Mauger said officials have also been reaching out to commercial vessels for help. 

The private company that operates the submarine, OceanGate Expeditions, said in a statement on Monday that it was "mobilizing all options" to rescue those on board. 

British billionaire Hamish Harding is among the passengers, according to a social media post from a relative.

Pakistani businessman Shahzada Dawood and his son, Suleman, were also on board, their family said in a statement. 

"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," the statement said. 

The US Coast Guard said earlier on Twitter that a boat on the surface - the Polar Prince - lost contact with the submarine, called the Titan, about one hour and 45 minutes after it began diving toward the site of the Titanic's wreckage on Sunday morning.

OceanGate said, "We are deeply thankful for the extensive assistance we have received from several government agencies and deep sea companies in our efforts to reestablish contact with the submersible." 

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

Harding himself had posted on Facebook that he would be aboard the sub. There have been no further posts from him. The expedition headed out to sea on Friday, and the first dive was set for Sunday morning, according to Harding's post. 

The statement from the Dawood family said the father and son had planned the journey to visit the remnants of the Titanic shipwreck. 

Dawood is the vice chairman of one of Pakistan's largest conglomerates, Engro Corporation, which has investments in fertilisers, vehicle manufacturing, energy and digital technologies. According to the website of SETI, the California-based research institute of which he is a trustee, he lives in Britain with his wife and two children.

The expeditions, which cost $250,000 per person, 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.

In order to visit the wreck, passengers climb inside Titan, the five-person submersible, which takes two hours to descend approximately 12,500 feet (3,800 m) to the Titanic.

The British passenger ship famously sank in 1912 on its maiden voyage after striking an iceberg, killing more than 1,500 people. The story has been immortalized in nonfiction and fiction books as well as the 1997 blockbuster movie "Titanic."

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