The mini-sub searching for missing flight MH370 has reached record depths well beyond its normal operating limits, officials said Friday as it dived on its fifth seabed mission.

With no results to show since the Boeing 777 carrying 239 people disappeared on March 8, Australia's Prime Minister Tony Abbott has set a one-week deadline to locate the plane which is believed to have crashed in a remote area of the Indian Ocean west of Perth.

Searchers have extended the hunt beyond the normal 4,500 metre (15,000 feet) depth range of the US Navy's Autonomous Underwater Vehicle (AUV) called Bluefin-21.

"The AUV reached a record depth of 4,695 meters during mission four," the US Navy said. "This is the first time the Bluefin-21 has descended to this depth.

"Diving to such depths does carry with it some residual risk to the equipment and this is being carefully monitored," a statement said.

Australia's Joint Agency Coordination Centre (JACC) announced that the mini-sub had been deployed on a new mission as operations run round the clock.

"Data analysis from the fourth mission did not provide any contacts of interest," it added.

The unmanned Bluefin-21 which maps the seafloor by sonar, has searched 110 square kilometres (43 square miles) to date, JACC said.

The UAV, which hit a technical snag on Tuesday had also re-surfaced Monday after breaching a pre-programmed maximum depth of 4.5 kilometres (2.8 miles).

JACC said Thursday night that the US manufacturer of the UAV, Phoenix International, had advised the risk was "acceptable".

"This expansion of the operating parameters allows the Bluefin-21 to search the sea floor within the predicted limits of the current search area," it said.

The Malaysia Airlines jet is believed to have crashed in the ocean after mysteriously vanishing while en route between Kuala Lumpur and Beijing.

Hopes for finding the plane have focused on the Bluefin-21 after signals believed to be from the plane's flight data recorders on the seabed fell silent in recent days.

The submersible is being deployed from an Australian vessel to scan an uncharted seafloor at extreme depths, but Abbott said the Bluefin-21 would be given about a week as questions are asked about the massive costs.

"If we don't find wreckage, we stop, we regroup, we reconsider," Abbott told the Wall Street Journal.

Both Abbott and Malaysia's Transport Minister Hishammuddin Hussein vowed not to give up looking for the plane.

- Huge costs building up -

However suggestions have emerged that more sophisticated -- and highly expensive -- deep-diving equipment may be needed for the search.

"We have to look at contractors, and the cost of that will be huge," Hishammuddin told reporters in Kuala Lumpur, though he indicated that such concerns were not yet testing the resolve of multinational search partners.

"But in any event, the search will always continue. It's just a matter of approach."

Analysts have told AFP the search will be the most expensive in aviation history, with Ravikumar Madavaram, an aviation expert at Frost & Sullivan Asia Pacific, estimating the bill at $100 million so far.

"This is definitely the biggest operation ever," he said. "In terms of costs this would be the highest."

Visual searches of the ocean surface have failed to find any floating debris, and the JACC said Thursday an analysis of samples from an oil slick found at the weekend had determined it was not from MH370.

"If the current search turns up nothing, we won't abandon it, we will simply move to a different phase," Abbott said.

JACC chief Angus Houston said earlier this week that alternatives, including devices that can go deeper than the Bluefin-21, were "being looked at", but he gave no specifics.

Houston has repeatedly warned the search will be protracted and demand patience, particularly from distraught families of passengers, who still have no confirmation of what happened to their loved ones.

MH370 has drawn increasing comparisons to the effort to locate the underwater resting place of Air France flight 447, which crashed in the Atlantic in 2009.

It took nearly two years for AF447's flight data recorders to be recovered.