On Monday, more than a month after the Malaysia Airlines Boeing 777 vanished, the Australian head of the search's Joint Agency Co-ordination Centre announced the deployment of an underwater vehicle.

The Bluefin-21 was due to be deployed from Australian defence vessel Ocean Shield on Monday night, Air Chief Marshal (retired) Angus Houston revealed.

The development came as news of a discovery broke: an oil slick had been found near the vicinity of the underwater search on Sunday and a sample had been sent for analysis, he said. But it would be days before the sample could be ''conclusively tested''.

''The source of the oil has yet to be determined,'' Air Chief Marshal Houston said. ''But the oil slick is approximately 5500 metres downwind and down sea from the vicinity of the detections picked up by the towed pinger locator on Ocean Shield.''

That pinger locator, which has garnered the search's best leads to date, four pings, believed to be from MH370's black boxes, was to be retired on Monday to make way for deployment of the Bluefin-21.

Abandoning the towed pinger locator search also suggested authorities believed the black box signals were likely dead.

It has been more than a week since the batteries from the flight's two black box beacons were due to expire, and six days since signals consistent with aircraft black box ''pings'' were last detected.

''Despite the lack of further detections the four signals previously acquired taken together constitute the most promising lead we have in the search for MH370,'' Air Chief Marshal Houston said.

The Bluefin-21 can operate at a maximum depth of 4½ kilometres and travels at a maximum speed of 4½ knots, or less than nine kilometres an hour, its manufacturer said.

Travelling at a walking pace, each search mission would take the Bluefin-21 at least 24 hours, with only 16 of those spent searching the ocean floor, Air Chief Marshal Houston said.

Creating a 3D image of ground, using side scanning sonar, the Bluefin-21 would cover an area of 40 square kilometres on its first day in operation.

It takes the vehicle two hours to reach the sea floor. It can then search for 16 hours before taking two hours to return to the surface. It takes four hours to download the data it has detected for analysis by a team of about 10 US contractors aboard the Ocean Shield.

The Bluefin-21 will be deployed to a couple of highly targeted areas, which were identified by expert analysis of the pings, within the broader underwater search, US Navy supervisor of salvage Captain Mark Matthews said.

He said the US operators of the Bluefin-21 would be looking for darker images in a monochromatic picture of the ocean floor. ''They're looking for things that appear abnormal, so man-made shapes,'' Captain Matthews said.

''We've got some indications that we've got some promising leads that we're going to pursue.''

Malaysia Airlines flight MH370, with 239 people on board, vanished after mysteriously changing course during a flight from Kuala Lumpur to Beijing on March 8.