A "debris field" has been discovered by an underwater robot searching near the wreck of the Titanic for a missing submersible with five people on board, rescuers said on Thursday.

The development came after rescuers insisted that the multinational mission to locate the craft was still focused on finding the crew alive despite fears that the vessel's oxygen may have run out.

"Experts within the unified command are evaluating the information," the US Coast Guard said in a tweet.

The coast guard said the debris field was found "within the search area by an ROV (Remotely Operated Vehicle) near the Titanic."

It did not give more details but said it would hold a press briefing at 3:00 pm (1900 GMT) in Boston.

Two more robots were deployed on Thursday in the hunt for the Titan sub, lost somewhere in a vast swathe of the North Atlantic between the ocean's surface and nearly four kilometres below.

Based on the sub's capacity to hold up to 96 hours of emergency air, rescuers had estimated that the passengers, which include fee-paying tourists, may have run out of oxygen in the early hours of Thursday.

But as that possible deadline passed, US Coast Guard Rear Admiral John Mauger said rescuers were "fully committed" to search operations.

"People's will to live really needs to be accounted for as well. We're going to continue searching," he told NBC's Today show.

A surge of assets and experts have joined the operation in the past day, and sonar has picked up unidentified underwater noises. (AFP)