Updated 1.40pm

Rescue teams raced against time on Tuesday in their search for a tourist submersible that went missing near the wreck of the Titanic with five people on board.

One of the passengers has been identified as British businessman Hamish Harding, whose aviation firm had posted on social media about his expedition.

Prominent Pakistani businessman Shahzada Dawood, who is vice-chairman of the conglomerate Engro, and his son Suleman were also on board, a family statement said.

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

"As of now, contact has been lost with their submersible craft and there is limited information available," the Dawood family statement said.

"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," it added.

The US Coast Guard had launched two planes to survey the remote area in the North Atlantic, while its Canadian counterparts had sent a plane and a ship.

Time is a critical factor. The vessel has a range of 96 hours for the crew of five, and US Coast Guard Rear Admiral John Mauger said Monday afternoon that he believed it still had 70 or more hours of oxygen remaining.

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

This image courtesy of Dirty Dozen Productions shows Hamish Harding ahead of the 4am start of the RMS Titanic Expedition Mission 5 on June 18.

But with no reported sightings of the vessel or communication signals throughout the day, the US Coast Guard halted its flights for the day.

It said search operations through the night would be led by the US National Guard and the mission's operator.

The Coast Guard added that searches by Canadian aircraft, which were using buoys to scan underneath the surface, would continue on Tuesday morning.

Rory Golden, an Irish explorer on a support vessel with OceanGate Expeditions, urged the public to avoid speculation, and thanked those offering support.

"The reaction and offers of help globally is truly astonishing, and only goes to show the real goodness in people at a time like this," he wrote in a Facebook post.  

An OceanGate Expeditions spokesperson told AFP in a statement late Monday that "for some time, we have been unable to establish communications with one of our submersible exploration vehicles which is currently visiting the wreck site of the Titanic."

"Our entire focus is on the wellbeing of the crew and every step possible is being taken to bring the five crew members back safely."

The company uses a submersible named Titan for its dives to the Titanic wreck, with seats priced at $250,000, according to its website.

Harding, a 58-year-old aviator, space tourist and chairman of Action Aviation, had posted Sunday on his Instagram account that he was proud to join OceanGate's Titanic mission.

"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," he wrote.

Paul-Henry Nargeolet, a veteran diver and expert on the Titanic wreck, was also part of the Titan crew, Harding said in his post.

"The team on the sub has a couple of legendary explorers, some of which have done over 30 dives to the RMS Titanic since the 1980s including PH Nargeolet," the post says.

Action Aviation posted Sunday on Twitter that "the sub had a successful launch and Hamish is currently diving," and included several photographs of Harding and mission staff on the surface.

- 'Clock is ticking' -The Titanic hit an iceberg and sank in 1912 during its maiden voyage from England to New York with 2,224 passengers and crew on board. More than 1,500 people died.

The wreckage is in two main pieces 400 miles off the coast of Newfoundland, Canada, some 13,000 feet underwater.

It was found in 1985 and remains a source of fascination and a lure for nautical experts and underwater tourists.

Without having studied the craft itself, Alistair Greig, professor of marine engineering at University College London, suggested two possible theories based on images of the vessel published by the press.

He said if it had an electrical or communications problem, it could have surfaced and remained floating, "waiting to be found."

"Another scenario is the pressure hull was compromised -– a leak," he said in a statement. "Then the prognosis is not good."

While the submersible may still be intact during its dive, "there are very few vessels" able to go to the depth to which the Titan might have traveled.

"The clock is ticking, and any submariner/submersible deep divers know how unforgiving the Abyssal domain is: going undersea is as, if not more, challenging than going into space from an engineering perspective," said University of Adelaide associate professor Eric Fusil in a statement.