"Banging sounds" have reportedly been heard during the underwater search for a submersible with five people on board that went missing during a dive to the Titanic wreck.
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CNN has reported an internal US government memo revealed sonar had picked up the noises on Tuesday local time.
A later internal update said "additional acoustic feedback was heard... also indicating continued hope of survivors", according to CNN.
The US Coast Guard confirmed in a tweet just after midnight that a Canadian P-3 aircraft had "detected underwater noises in the search area".
"As a result, [remotely operated vehicle] operations were relocated in an attempt to explore the origin of the noises. Those ROV searches have yielded negative results but continue," the tweet said.
Canadian research vessel Polar Prince lost contact with the submersible 'Titan' about an hour and 45 minutes after it began a dive towards the wreck site in the Atlantic Ocean on Sunday local time.
US Coast Guard captain Jamie Frederick earlier told reporters crews were "working around the clock" in an effort to locate the Titan.
"We know there's about 40 hours of breathable air left based on the initial report of 96 hours from when the vessel submerged," he said.
He said a group of the "nation's best experts" were part of the response.
Since being notified on Sunday local time the search effort has covered nearly 20,000 square kilometres.
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"To date, those search efforts have not yielded any results," Captain Frederick said.
It has been a coordinated response from the US Coast Guard, US Navy, Canadian Armed Forces and Coast Guard and the Titan's parent company OceanGate Expeditions.
"Our thoughts and prayers are with the crew and their families and loved ones.
"We will provide unwavering effort as we continue the search."
The search area is 1500 kilometres east of Cape Cod, Massachusetts, and 650 km south of St John's, Newfoundland.
In a statement on June 20 OceanGate Expeditions said it was "mobilising all options to bring the crew back safely".
"We are working toward the safe return of the crewmembers."
British billionaire Hamish Harding is believed to be among the passengers according to a social media post before the sub's descent.
With Australian Associated Press
