A friend of one of the missing men on the submarine with five people on board says it will be a miracle if they survive.
But the search team looking for the submarine remain "optimistic and hopeful" despite the vessel only having around 20 hours of oxygen left.
The vessel lost contact late last week (local time), about an hour and 45 minutes after it departed for an expedition to the Titanic wreckage. The search team is currently focusing on an area where banging noises have been detected, although they can't confirm if they're coming from the missing submarine.
US Coast Guard Cpt James Frederick told media on Thursday morning (NZ time) a Canadian P-3 search plane detected the noises underwater in the search area on Tuesday and crews have been focused on finding the origin of the sounds. He also said a plane heard the noises on Wednesday as well.
'Miracle' if crew survive - friend
The five missing men on board the Titan submarine - with one pilot and four "mission specialists" onboard - have been identified.
They're Hamish Harding - a UK billionaire and chairman of aviation company Action Aviation - Shahzada Dawood, 48, and his son Suleman, 19, Paul-Henri Nargeolet - 77-year-old French explorer - and Stockton Rush - the founder and chief executive of the vessel's US-based operating company OceanGate.
A friend of Harding said "every single minute feels like hours" and it would be a "miracle" if the crew survived.
Norwegian explorer Jannicke Mikkelsen said she has been left unable to sleep and added she last spoke to Harding right before his dive to the Titanic and wished him "godspeed".
"I didn't consider that this type of expedition would be as dangerous as it's turned out to be," she said.
"As explorers, we are pessimistic and objective. And, as it stands right now, it would be a miracle if they are recovered alive."
She knew Harding would be "calm" amid the crisis but her biggest fear was knowing they're trapped "without being able to get help".
"There is no one who can reach him on the bottom," she said.
'One-hundred percent' still a search-and-rescue mission - US Coast Guard
Newshub US correspondent Mitch McCann asked Frederick at the press conference on Thursday how much hope he can give the families of the five missing men after hearing the noises.
Frederick responded by saying he didn't know what the noises are but said they were searching the area of the noises
"We need to have hope but I can't tell you what the noises are. But what I can tell you is and I think this is the most important point, we're searching where the noises are and that's all we can do at this point."
Carl Hartsfield, of the Woods Hole Oceanographic Institution, told media at the press conference the noises have been described as banging sounds, but also said it was difficult to discern the source of noises underwater.
"They have to put the whole picture together in context and they have to eliminate potential manmade sources other than the Titan," Hartsfield said.
The search area is continuing to expand, with it currently being 26,000sqkm wide and up to 4km deep, which is about "two times the size of Connecticut", Frederick said. The Coast Guard is bringing in more ships and underwater vessels to search for the missing submersible after detecting underwater sounds.
He confirmed the search for the missing submersible is "100 percent" still a "search-and-rescue mission" but time is of the essence with an estimated 20 hours of oxygen left.
The five passengers on board the missing Titan sub had 96 hours of breathable air, according to its operator OceanGate's specifications. This means they're down to about 20 hours of oxygen and could run out by Thursday evening (NZ time).
However, experts have said the air supply depends on a range of factors.
"We'll continue to put every available asset that we have in an effort to find the Titan and the crew members," Frederick said.
He said the search and rescue team has two remotely operated underwater vehicles "actively searching", plus several more are on the way and expected to join the search operation.
But even if the submersible is located, retrieving it presents huge logistical challenges, given the extreme conditions miles below the surface.
The 6.7m submersible began its descent at 8am on Sunday (local time). It lost contact with its parent surface vessel during what should have been a two-hour dive to the Titanic.
The wreck of the British ocean liner, which sank when it hit an iceberg on its maiden voyage in 1912, lies on the seabed at a depth of about 3810m. It is about 1450km east of Cape Cod, Massachusetts, and 643km south of St John's, Newfoundland.
The tourist expeditions, which cost US$250,000 per person, start in St John's before heading out approximately 640km into the Atlantic to the wreckage site, according to OceanGate's website.
