Rescuers are continuing their frantic search for a submersible that went missing while on a tour to the wreck of the Titanic at the bottom of the Atlantic.

US and Canadian coast guards and other personnel are racing against time as the submersible could run out of oxygen on Thursday morning local time.

The submersible set off to its undersea destination on Sunday, with five people on board. It lost contact with the surface less than two hours after starting its dive.

Rescue personnel say underwater noises were detected on Wednesday again after being first heard on Tuesday.

They are deploying remotely operated unmanned underwater probes in areas where the sounds were detected.

But the British public broadcaster BBC quoted a former US Navy nuclear submarine commander as warning that the noise may not be coming from the submersible.

The former commander, David Marquet, told the BBC that it "could just be natural sounds." He added that more ships are coming into the area, so more noises are being heard.

The BBC also said deep-sea explorer David Gallo believes it would take a miracle to rescue those trapped.

Gallo was quoted as saying that "it takes hours" to locate the submersible and then to get it up to the surface.