Dhaka: The huge search for a missing submersible vessel near the wreck of the Titanic has entered a critical stage, as fears loom over the oxygen levels that may be on board. If the sub is still functional and intact, it may only have low levels of oxygen left based on earlier estimates.

It went missing in a remote area of the North Atlantic on June 18 with a four-day oxygen supply for its crew of five. On June 22, many questions remained over how it could be recovered.

The minivan-sized submersible, which was owned and operated by the private company OceanGate Expeditions, is yet to be located. If it is found, it will need to be reached by complex rescue equipment and then brought to the surface in an operation that would likely take hours. That would need to happen before the oxygen supply runs out and without damaging its structure or endangering those on board.

On board the 21-foot vessel are British businessman Hamish Harding, British-Pakistani businessman Shahzada Dawood and his son Suleman, former French navy diver Paul-Henry Nargeolet and the CEO of OceanGate—which operates the submersible—Stockton Rush.

The condition of the vessel and its crew of five is unknown, but the US Coast Guard said the operation remains a rescue mission.

"This is a search-and-rescue mission, 100 per cent," Captain Jamie Frederick of US Coast Guard said on June 21.

British businessman Hamish Harding, British-Pakistani businessman Shahzada Dawood and his son Suleman, former French navy diver Paul-Henry Nargeolet and OceanGate CEO Stockton Rush are onboard the vessel

Earlier, officials said undersea noises had been detected by Canadian search planes on June 20 and 21. However, it is still unclear what these were, and officials said they may not have come from the submersible.

Remote-controlled underwater search vehicles (ROVs) were deployed to the area where the sounds were detected but are yet to find anything.

Several more ROVs were expected to arrive at the site on June 22 along with more multi-national support.

Tracking data showed the French research ship, Atalante, which is carrying a specialist deep-sea diving robot, nearing the area on June 22 morning. That robot is capable of researching depths below the Titanic wreck, which lies about 12,500 ft (3,810m) below the surface and has experience of surveying the Titanic.

The overall area of sea being scoured is about 26,000 sq km (10,000 sq miles), twice the size of the US state of Connecticut. The area is prone to stormy conditions and poor visibility which makes search operations more challenging, experts said.