Submarine exploring Titanic wreck declared missing

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 says an expedition to the Titanic site was "currently underway."

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 said there were five people on the vessel.

OceanGate was not immediately reachable.

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 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.

The wreckage is in two main pieces 400 miles off the coast of Newfoundland, Canada, under 12,500 feet (3,800 meters) of water. It was found in 1985 and remains a source of fascination and a lure for nautical experts and underwater tourists.