* Search focused on 10 km radius of sea floor
* Search in narrowed area to last about a week
* Malaysia says search at "very critical juncture"
* US Navy robot sub testing record depth limits
By Matt Siegel and Byron Kaye
SYDNEY/PERTH, Australia, April 19 (Reuters) - The current underwater search for missing Malaysia Airlines Flight MH370, focused on a tight 10 km (6.2 mile) circle of the sea floor, could be completed within a week, Australian search officials said on Saturday.
Malaysia said the search was at a "very critical juncture" and asked for prayers for its success.
A U.S. 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.
After almost two months without a sign of 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, officials said.
"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 told Reuters in an email.
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 that the underwater search were to prove fruitless.
More than two dozen countries have been involved in the hunt for the Boeing 777 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 such 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," Malaysian Acting Transport Minister Hishammuddin Hussein told a media conference in Kuala Lumpur, asking for people to pray for success.
Malaysia was asking oil companies and others in the commercial sector to provide assets that might help in the search, Hishammuddin added, after earlier saying more AUVs might be used.
DRONE GOES DEEPER THAN EVER BEFORE
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 U.S. 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 km (1,200 miles) northwest 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 (14,763 feet), authorities have adjusted the mechanism and have sent it as deep as 4,695 metres (15,403 feet), a record for the machine.
But hopes that it might soon guide searchers to wreckage are dwindling with no sign of the plane after six deployments spanning 133 square kilometres (83 square miles). Footage from the drone's sixth mission was still being analysed, the Joint Agency Coordination Centre said on Saturday.
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 on Saturday the Joint Agency Coordination Centre said up to 11 military aircraft and 12 ships would help with the Saturday's search covering about 50,200 square kilometres (31,000 square miles) across three areas.
"The search will always continue," Hishammuddin said. "It's just a matter of approach." (Editing by Lincoln Feast and Nick Macfie)