US and Canadian ships and planes are searching for a submarine that went missing off the coast of southeastern Canada while taking tourists to explore the wreckage of the Titanic, officials have said.
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 1,450km east of Cape Cod, some dropping sonarbuoys that can monitor to a depth of 3,962m, US Coast Guard Rear Admiral John Mauger told reporters.
"It is a remote area and it is a challenge to conduct a search in that remote area," Mr 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, adding, "...we will continue to fly aircraft and move additional vessels".
Mr 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 that it was "mobilising all options" to rescue those on board. British billionaire Hamish Harding is among the passengers, according to a social media post from a relative.
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."
Mr Harding's stepson wrote on Facebook that Mr 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.
Mr 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 Mr Harding's post.
The expeditions, which cost $250,000 per person, start in St John's, Newfoundland, before heading out approximately 640km into the Atlantic to the wreckage site, according to OceanGate's website.
In order to visit the wreck, passengers climb inside Titan, a five-person submersible, which takes about two hours to descend to the Titanic.
The Titanic famously sunk in 1912 on its maiden voyage to the US after striking an iceberg. More than 1,500 people perished in the disaster.
The story has been immortalised in non-fiction and fiction books as well as the 1997 blockbuster film Titanic.
The wreckage is in two main pieces 400 miles off the coast of Newfoundland, Canada, under 12,500ft of water.
It was found in 1985 and remains a source of fascination and a lure for nautical experts and underwater tourists.
