Rescuers searching for a missing submersible near the wreck of the Titanic have detected "underwater noises" in the search area, the US Coast Guard said on Wednesday, with the five on board estimated to have less than 24 hours of oxygen left.

All communication was lost with the 6.5-metre craft during its descent on Sunday to see the remains of the British passenger liner, which sits nearly four kilometres below the surface of the North Atlantic.

US and Canadian coast guard ships and planes are scouring around 20,000 square kilometres of ocean – larger than the US state of Connecticut – for the vessel, which was attempting to dive about 640 kilometres off the coast of Newfoundland, Canada.

"Canadian P-3 aircraft detected underwater noises in the search area. As a result, ROV (remotely operated vehicle) operations were relocated in an attempt to explore the origin of the noises," the US Coast Guard's First District said on its official Twitter page.

The ROV searches had so far not yielded results, but would continue, the military branch said, adding that data from the Canadian aircraft had been shared with US Navy experts to inform future search plans.

The submersible, named Titan, was carrying three fee-paying passengers, including a British billionaire and a Pakistani tycoon and his son. OceanGate Expeditions charges US$250,000 for a seat on the sub, which is about the size of an average truck.

The US Coast Guard announcement is the most encouraging sign yet that those on board may still be alive.

Rescue aid has been pouring in from around the world, with a specialised winch system for lifting heavy objects from extreme depths, other equipment and personnel due to join the rescue effort on Tuesday night, according to the US Navy.

The Pentagon said it was deploying a third C-130 aircraft and three C-17s, while France's oceanographic institute announced a deep-sea underwater robot and its experts would arrive in the area on Wednesday.

"This is a very complex search and the unified team is working around the clock to bring all available assets and expertise to bear as quickly as possible," US Coast Guard Captain Jamie Frederick told reporters.

Rescuers estimate that passengers now have less a day of oxygen left, based on the sub's capacity to hold up to 96 hours of emergency air. (AFP)