An underwater robotic submarine is expected to finish searching a narrowed down area of the Indian Ocean seabed for the missing Malaysia Airlines plane within the next week, after completing six missions so far, the search coordination centre has said.

As the hunt for Flight 370 hits week six, the Bluefin 21 unmanned machine began its seventh trip into the depths off the coast off western Australia.

Its search area forms a 10-kilometre circle around the location of an underwater signal that was believed to be coming from the aircraft's black boxes before the batteries died.

The sonar scan of the seafloor in that area is expected to be completed in five to seven days, the centre said in an email to the AP news agency on Saturday.

The US navy submarine has covered around 133 square kilometres since it began diving into the depths on March 14.

The latest data are being analyzed, but nothing has yet been identified.

No wreckage

On Saturday, up to 11 aircraft and 12 ships continued to scan the ocean surface for debris from the Boeing 777 that disappeared en route from Kuala Lumpur, Malaysia, to Beijing with 239 people on board.

Radar and satellite data show the plane mysteriously veered far off-course for unknown reasons and would have run out of fuel in the remote section of the southern Indian Ocean where the search has been focused.

Not one piece of debris has been recovered since the massive search effort began, AP reported.

The search, which continues to raise more questions than answers, has tormented the families whose loved ones never came home on March 8. About two-thirds of the passengers were Chinese.

There have been numerous leads throughout the painstaking hunt, but all have turned out to be false.

Oil slick

The latest hope involved an oil slick found near the underwater search area, but analysis of a sample taken from the site found it was not connected to the plane.

The most promising development came when four underwater signals were detected on April 5 and 8.

The sounds were consistent with pings that would have been emanating from the flight data and cockpit recorders' beacons before their batteries died.

The underwater operation is being complicated by the depth of the largely unexplored silt-covered sea floor.

The US submarine has gone beyond its recommended limit of 4,500 metres, according to the US 7th Fleet. That could risk the equipment, but it is being closely monitored.

The search coordination centre has said the hunt for floating debris on the surface will continue at least into next week, even though head of the search effort Angus Houston had earlier said it was expected to end sooner.

Source: Agencies