Australia: 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 2,000km (1,200 miles) 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 (14,763 feet). Its searches have yet to find any sign of the plane.

On Friday, 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 (15,403 feet) 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 in a statement.

“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 also wrong and the drone was focusing on a “reduced and more focused underwater search area”.

Malaysia and Australia will sign a deal specifying who handles any wreckage from missing flight MH370 that may be recovered, including the crucial “black box” flight data recorders, local media reported on Friday.

Malaysia is drafting the agreement “to safeguard both nations from any legal pitfalls that may surface during that (recovery) phase,” the New Straits Times reported.

The government hopes the deal can be finalised soon and endorsed in a Cabinet meeting next week. Canberra is studying the memorandum of understanding, it said.

“The MoU spells out exactly who does what and the areas of responsibility,” civil aviation chief Azharuddin Abdul Rahman was quoted as saying.

Azharuddin added that Malaysia would lead most of the investigation, with Australia and others helping. Details of the MoU will not be made public, the report said.

Azharuddin and other officials could not immediately be reached for comment.