Boston, Massachusetts - Experts are searching for a missing submersible headed for the Titanic wreck in the area where noises have been heard beneath the surface, the US Coast Guard confirmed. Rescue efforts have intensified amid the race against the clock.

Captain Jamie Frederick told reporters in Boston on Wednesday the cause of the noises in the North Atlantic was still unconfirmed but insisted the search efforts to find the five missing people aboard the Titan submersible is still "a rescue mission."

Rescue workers are racing to beat a rapidly closing oxygen window as they hunt for a missing submersible near the wreck of the Titanic, after noises detected by sonar raised hopes the five people onboard are still alive.

The US Coast Guard said it had not identified whether the source of the sounds was indeed the small sub, whose passengers are estimated to have less than 20 hours of oxygen left.

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"We don't know what they are, to be frank with you," Captain Jamie Frederick said of the underwater noises picked up late Tuesday by a Canadian aircraft. But he added: "We have to remain optimistic and hopeful."

"The surface search is now approximately two times the size of Connecticut, and the sub-surface search is up to 2.5 miles deep, exponentially expanding the size of the search area," Frederick added.

Extra ships, specialized salvage equipment and US Navy experts converged on the vast search area in the North Atlantic as rescuers, who have received help from around the world, concentrated their efforts near the sounds.

Captain Frederick said: "we’re searching in the area where the noises were detected, and we’ll continue to do so," adding that additional "ROVs will be there in the morning" to help.

Carl Hartsfield, from the Woods Hole Oceanographic Institution, told reporters that the the noises that have been described as "banging noises" are being analyzed by "the best people in the world."

He added: "The ocean is a very complex place, obviously human sounds, nature sounds, and it’s very difficult to discern what the sources of those noises are at times, but I can tell you that this team has multiple sensors that are in the area, they’re sending data back expeditiously to the best people in the world to analyze that data and they’re feeding the results of the analysis back to the unified team and they’re making decisions."

The vessel lost communication with tour operators on Sunday while about 435 miles south of St John’s, Newfoundland, during a voyage to the Titanic shipwreck off the coast of Canada.

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The 22-feet-long OceanGate Expeditions vessel, which has British billionaire adventurer Hamish Harding on board, may have less than 20 hours of oxygen left.

Also in the undersea craft are UK-based businessman Shahzada Dawood, his son Suleman Dawood, and OceanGate’s chief executive and founder Stockton Rush, reportedly with French submersible pilot Paul-Henri Nargeolet.

On Titan’s dwindling oxygen levels, Captain Frederick said: "Oxygen is just one piece of data. There are lots of pieces of data that we need to study. But (oxygen) is not the only thing that’s important."

The vessel reportedly had a 96-hour oxygen supply in case of emergencies.

An object spotted in the ocean by an aircraft during the search for Titan is not believed to be debris, Capt Frederick said.