Boston, Massachusetts – A submarine vessel used for taking tourists to see the wreckage of the Titanic in the North Atlantic has gone missing, triggering a search-and-rescue operation, the US Coast Guard said Monday.

It was not immediately known how many people are on the vessel, operated by a company called OceanGate Expeditions, but a British aviator was believed to be onboard.

"Yes, we're searching for it," said an official from the US Coast Guard Rescue Coordination Center in Boston.

On its website, OceanGate Expedition said an expedition to the Titanic site was "currently underway." The company was not immediately reachable, and its website currently shows a broken link.

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In a statement quoted by CBS News and other media outlets, OceanGate Expeditions said: "Our entire focus is on the crewmembers in the submersible and their families."

British billionaire and aviator Hamish Harding had on Sunday posted on his Instagram account that he was "proud to finally announce" he had joined the OceanGate Expedition "for their RMS TITANIC Mission as a mission specialist on the sub going down to the Titanic."

"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," added the 58-year-old aviator.

"A weather window has just opened up and we are going to attempt a dive tomorrow."

It was not known who else was onboard. The Guardian newspaper reported there were five people on the vessel.

In the statement it added that it was "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."

The Canadian Coast Guard said it is also taking part in the search effort with a fixed wing plane and a ship sent to the search area.

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 in the tragedy, and it has become the most famous ocean liner accident.

The wreckage is in two main pieces 400 miles off the coast of Newfoundland, Canada, under 12,500 feet of water. It was found in 1985 and remains a source of fascination and a lure for nautical experts and underwater tourists.