Media playback is unsupported on your device Media caption ACM Angus Houston: Bluefin-21 will create a 3D map of the ocean floor

Teams searching for the missing Malaysia Airlines plane are to deploy a robotic submarine for the first time.

Search chief Angus Houston said the Bluefin-21 drone would be sent down as soon as possible to search for wreckage on the sea floor.

Teams have been using a towed pinger locator to listen for signals from the plane's "black box" flight recorders.

But no new signals have been heard since 8 April, amid concerns the flight recorders' batteries have expired.

Flight MH370 went missing on 8 March with 239 people on board. It was flying from Kuala Lumpur to Beijing when it lost contact with air traffic controllers over the South China Sea.

Malaysian officials believe, based on satellite data, that it ended its flight in the southern Indian Ocean, thousands of kilometres off course.

'Manageable search area'

An international search has focused on waters west of the Australian city of Perth, with teams racing against time to detect signals before the flight recorder batteries - which last about one month - run out.

Image copyright Getty Images Image caption The Bluefin-21 will search the sea floor for wreckage from flight MH370

Image copyright Reuters Image caption Up to 12 planes and 15 ships were involved in the search for the plane

Air Chief Marshal Houston, who heads the joint agency co-ordinating the search effort, said that given no signals had been detected in six days, it was time to go underwater.

The Bluefin-21 - an almost 5m-long underwater autonomous vehicle that can create a sonar map of the sea floor - will search for wreckage in an area defined by four signals heard last week.

Officials believe those signals - picked up by the pinger locator towed by an Australian vessel - are consistent with flight recorders.

"Analysis of the four signals has allowed the provisional definition of a reduced and manageable search area on the ocean floor," ACM Houston said.

"The experts have therefore determined that the Australian Defence Vessel Ocean Shield will cease searching with the towed pinger locator later today and deploy the... Bluefin-21 as soon as possible.''

He warned that the submersible search would be a long, "painstaking" process that might, in the end, yield no results.

Each Bluefin-21 mission will last 24 hours, with 16 hours spent on the ocean floor, four hours' diving and resurfacing time, and four hours to download data.

MH370 - Facts at a glance 8 March: Malaysia Airlines Kuala Lumpur-Beijing flight carrying 239 people disappears

Malaysia Airlines Kuala Lumpur-Beijing flight carrying 239 people disappears Plane's transponder, which communicates with ground radar, was switched off as it left Malaysian airspace

Satellite 'pings' indicate plane was still flying seven hours after satellite contact was lost

24 March: Based on new calculations, Malaysian PM says "beyond reasonable doubt" that plane crashed in southern Indian Ocean with no survivors What we know The search for flight MH370

The Ocean Shield had also spotted an oil slick in the same area where the signals had been heard, ACM Houston said, and a sample was being sent for testing.

"I stress the source of the oil is yet to be determined but the oil slick is approximately 5,500m downwind... from the vicinity of the detections picked up by the towed pinger locator," he said.

Australian officials have said previously that they are confident they are searching in the right area for the missing plane.

Officials have no idea yet why the plane diverted so far from its intended flight path. Investigators are looking at options including hijacking, mechanical failure, sabotage and pilot action.

But recovering the flight recorders is seen as key to understanding what happened to the plane.