The current underwater search for missing Malaysia Airlines flight MH370, focused on a tight 10-kilometre circle of the sea floor, could be completed within a week, Australian search officials say.

Malaysia, meanwhile, says the search is at a "very critical juncture" and has asked for prayers for its success.

A United States Navy deep-sea autonomous underwater vehicle (AUV) is scouring a remote stretch of the Indian Ocean floor for signs of the plane, which disappeared from radars on March 8 with 239 people on board, including six Australians.

What is the Bluefin-21? An autonomous underwater vehicle (AUV) designed for deep-sea surveying.

An autonomous underwater vehicle (AUV) designed for deep-sea surveying. It has a "swappable payload". It will first use sonar in the search and will be refitted with cameras if something is detected.

It has a "swappable payload". It will first use sonar in the search and will be refitted with cameras if something is detected. It's 5m long and weighs 750kg. Has an endurance of 25 hours underwater at a speed of 3 knots, with a top speed of 4 knots.

It's 5m long and weighs 750kg. Has an endurance of 25 hours underwater at a speed of 3 knots, with a top speed of 4 knots. It has a depth rating of 4,500m, meaning it will be at its limit in the Indian Ocean search zone.

It has a depth rating of 4,500m, meaning it will be at its limit in the Indian Ocean search zone. Bluefin Robotics says its AUV can also be used for archaeology, oceanography, mine countermeasures, and unexploded ordnance.

After almost two months without finding any wreckage, the current underwater search has been narrowed to a small area around the location in which one of four acoustic signals - believed to be from the plane's black box recorders - was detected on April 8.

"Provided the weather is favourable for launch and recovery of the AUV and we have a good run with the serviceability of the AUV, we should complete the search of the focused underwater area in five to seven days," the Joint Agency Coordination Centre (JACC) said.

Officials did not indicate whether they were confident that this search area would yield any new information about the flight, nor did they state what steps they would take in the event the underwater search proved fruitless.

More than two dozen countries have been involved in the hunt for the Boeing 777 which disappeared from radar shortly into a Kuala Lumpur-to-Beijing flight in what officials believe was a deliberate act.

Weeks of daily sorties have failed to turn up any trace of the plane, even after narrowing the search to an arc in the southern Indian Ocean, making this the most expensive search operation in aviation history.

"It is important to focus on today and tomorrow. Narrowing of the search area today and tomorrow is at a very critical juncture," Malaysia's acting transport minister, Hishammuddin Hussein, told a media conference in Kuala Lumpur.

Malaysia was asking oil companies and others in the commercial sector to provide assets that might help in the search, Mr Hishammuddin added, after earlier saying more AUVs might be used.

Unmanned submarine dives deeper than ever

After almost two weeks without picking up any acoustic signals, and long past the black box battery's 30-day life expectancy, authorities are increasingly reliant on the $4 million US Bluefin-21 drone, which on Saturday was expected to have dived to unprecedented depths.

Because visual searches of the ocean surface have yielded no concrete evidence, the drone, with its ability to search deep beneath the ocean surface with "side scan" sonar, has become the focal point of the search 2,000 kilometres north-west of Perth.

The search has so far centred on a city-sized area where a series of "pings" led authorities to believe the plane's black box may be located. The current refined search area is based on one such transmission.

After the drone's searches were hampered by an automatic safety mechanism which returns it to the surface when it exceeds a depth of 4.5 kilometres, authorities have adjusted the mechanism and have sent it as deep as 4,695 metres - a record for the machine.

The drone has so far been deployed six times, spanning 133 square kilometres.

Footage from the drone's sixth mission was still being analysed, the JACC said.

Reuters