With only around three days of reserve oxygen estimated to be left onboard, each hour has now become crucial, to locate the tourist submersible that went missing in the North Atlantic while voyaging to the wreck of the Titanic.

OceanGate Expeditions, a Washington-based deep-sea exploration company, owners of the submersible, lost contact with the submersible on Sunday morning.

United States and Canadian rescue teams were involved in the search for the submersible carrying five people. According to Rear Adm. John Mauger, commander of the First Coast Guard District, the authorities are still trying to identify the five people onboard.

The Coast Guard on Monday said that there was a pilot and four "mission specialists" aboard.

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This undated photo provided by OceanGate Expeditions in June 2021 shows the company's Titan submersible | AP

Two aircraft from the US and Canada were involved in the search, along with a commercial ship, and further assets will be added as the pursuit continues into the night, said Mauger at a news conference.

“The operation's location- about 900 miles wast of Cape Cod and up to 13,000 feet deep- complicates the task, as does the need to look both on the surface of the water and below. It is a challenge to conduct a search in that remote area,” he said.

"But we are deploying all available assets to make sure that we can locate the craft and rescue the people on board," he added.

The Coast Guard said that two searches by C-130 aircraft had been completed and the mission would continue through the night.

21-foot submersible named "Titan", which left from St. John's, began its dive Sunday morning.

Reportedly, the Polar Prince, the Canadian ship supporting the watercraft, lost contact with it about an hour and 45 minutes later.

The company's expeditions to the Titanic wreck site include archaeologists and marine biologists. Oceangate also brings people who pay to come along, known as "mission specialists".

"We are deeply thankful for the extensive assistance we have received from several government agencies and deep sea companies in our efforts to re-establish contact with the submersible," Oceangate said. "We are working toward the safe return of the crew members."

Mauger said that the submersible has a 96-hour emergency sustainment capability, that includes the oxygen and fuel. "So we anticipate that there's somewhere between 70 and full 96 hours available at this point," he said.

The Titanic wreckage is believed to lie 12,500 feet deep.

Meanwhile, Action Aviation through a social media post said that its company chairman British businessman Hamish Harding, based in United Arab Emirates, was one of the travellers onboard.

Before starting the expedition he had wrote that "I am proud to finally announce that I joined OceanGate Expeditions for their RMS TITANIC Mission as a mission specialist on sub going down to the Titanic."

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