A French deep-diving robot is being rushed to the site of the missing sub, but won't arrive until Wednesday evening

insider: A robot that can go to depths of almost 20,000 feet below sea level is en route to join the search for the submersible that went missing near the site of the Titanic wreckage on Sunday, but it won't arrive until the crew's oxygen has nearly run out.

Hervé Berville, the French secretary of state for the sea, said the research vessel Atlante was on its way to the rescue site, French newspaper Il Figaro reported.

The Atlante is equipped with a remote operated vehicle, Victor 6000, which is operated by cable and "carries out observation and handling tasks to a depth of 6,000 metres [19,685 feet]," according to the French Research Institute for Exploitation of the Sea.

Berville said it was expected to arrive by 8pm Wednesday.

The vessel would likely arrive at the search area just hours before the oxygen available in the Titan submersible runs out. The US Coast Guard said the crew's oxygen supplies are likely to run out by Thursday afternoon.

"The idea and our goal is for the ship to arrive as soon as possible," Berville said, according to The Telegraph, adding: "This is a race of time."

OceanGate Expeditions' submersible lost contact with its mothership less than two hours into its journey to the Titanic, the US Coast Guard said on Monday. Five people were on board, including OceanGate's founder and CEO Stockton Rush, according to reports.

It is not clear if the submersible is intact or if the people on board are still alive.

Experts say there are three likely scenarios for what has happened: the submersible could already be on the surface, waiting to be discovered but unable to communicate for some reason; the submersible could have cracked and imploded; or it could be tangled in the wreckage of the Titanic.

A desperate search is underway, involving a variety of equipment and vessels, including a submarine, aircraft, and remote operated vehicles.

The Titanic's wreckage is located around 13,000 feet below sea level — thousands of feet deeper than where any undersea rescue has ever taken place before. That could make rescue difficult even if the submersible is located.

Other vessels that could be key to any rescue attempt are also heading to the site.

This includes a Royal Canadian Navy ship that has a six-person mobile hyperbaric recompression chamber on board, which could help deal with decompression sickness of any survivors, the BBC reported.

That craft is expected to arrive by midday on Thursday.