The Bluefin-21 Autonomous Underwater Vehicle during the search for the missing Malaysian Airlines flight MH370 yesterday. Photo: Reuters

An underwater drone scouring the Indian Ocean floor for a missing Malaysian jetliner has dived to its deepest ever level, putting its equipment at unprecedented risk, as hopes dwindled that it might soon turn up some sign of wreckage.

The US Navy’s Bluefin-21 and its “side scan” sonar has become the focal point of the search some 2,000 kilometres west of the Australian city of Perth, where authorities believe Malaysia Airlines flight MH370 hit the ocean after disappearing from radars on March 8 with 239 people on board.

The search has centred on a city-sized area where a series of “pings” led authorities to believe the plane’s black box may be located. But after more than a week without a signal, and almost two weeks past the black box battery’s life expectancy, authorities have turned to the Bluefin-21.

But the Bluefin-21’s searches of the largely unmapped ocean floor have been frustrated by an automatic safety mechanism which sends it to the surface when it exceeds a depth of 4.5 kilometres.

Yesterday, as searchers waited for the remote-control submarine to return from its fifth mission, the US Navy said the Bluefin-21 had gone to a record depth of 4,695 metres in its previous mission.

“This is the first time the Bluefin-21 has descended to this depth,” US Navy spokesman Lieutenant Junior Grade Daniel S. Marciniak said.

“Diving to such depths does carry with it some residual risk to the equipment and this is being carefully monitored by the US Navy and (Bluefin-21 owner) Phoenix International.”

He also confirmed that the Bluefin-21’s search area had been reduced based on further analysis of the initial signals believed to have come from the plane’s black box.

Authorities have said the US Navy’s previous estimate, that the Bluefin-21’s hunt may take two months, was wrong as the drone is focusing on a “reduced underwater search area”.