The search for missing Malaysia flight MH370 is at a "critical juncture", according to Malaysia's transport minister.

Hishammuddin Hussein's comments come two days after the Australian Prime Minister said a submersible device, Bluefin-21, had one week to find wreckage from the plane in the Indian Ocean.

"The search for today and tomorrow is at a critical juncture. We appeal for everybody around the world to pray that we find something," Mr Hussein said. 

Meanwhile, Australian officials supervising the search for the missing flight have said an underwater search for the black box recorder based on "pings" possibly from the device could be completed in five to seven days.

A US Navy deep-sea autonomous underwater vehicle is scouring a remote stretch of the Indian Ocean floor for signs of the flight.

The plane disappeared from radars on 8 March with 239 people on board and is believed to have crashed in the area.

The current underwater search has been narrowed to a circular area with a radius of 10km around the location in which one of four pings believed to have come from the black box recorders was detected on 8 April, officials said.

International search effort has so far proved fruitless

The massive international search and rescue effort for any physical evidence of the plane's wreckage, now in its seventh week, has so far proved fruitless.

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

After almost two weeks without a signal, and long past the black box battery's 30-day life expectancy, authorities are increasingly reliant on the Bluefin-21 drone, which was expected to dive to unprecedented depths today.

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

The search has thus 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 frustrated by an automatic safety mechanism which returns it to the surface when it exceeds a depth of 4.5 km authorities have adjusted the mechanism and have sent it as deep as 4,695 metres.

However, hopes that it might soon guide searchers to wreckage are dwindling with no sign of the plane after six deployments spanning 133 square kilometres.

Footage from the drone's sixth mission was still being analysed, the Joint Agency Coordination Centre said today.

Malaysian acting Transport Minister Datuk Seri Hishammuddin Hussein said in a Twitter post that the government's Deployment of Assets Committee was considering using more autonomous underwater vehicles, a possible sign of growing confidence in the vessels.

He did not elaborate.

On Monday, the search coordinator, retired Air Chief Marshal Angus Houston, said the air and surface search for debris would likely end by midweek as the operation shifted its focus to the ocean floor.

But the air and surface searches have continued daily, and today the Joint Agency Coordination Centre said up to 11 military aircraft and 12 ships would help with the day's search covering about 50,200 square kilometres across three areas. 

Hallmarks of a 'depressurisation incident'

Meanwhile, the Chief Executive of Ryanair, Michael O'Leary, has said the circumstances surrounding the Malaysian Airline plane, has all the hallmarks of a "depressurisation incident."

Michael O'Leary said the plane seemed to fly for quite a while until it ran out of fuel, which is what tends to happen in such an incident. 

Mr O'Leary said "nobody in the industry can understand why the transponder was turned off" and that such an operation cannot be done easily as it requires a certain degree of knowledge. 

He said he expects airlines and the authorities will now respond by making sure that the transponder cannot be disconnected in future.