PERTH, April 14, 2014 (AFP) - Australia was Monday deploying a mini-sub to scour the Indian Ocean seabed for missing Malaysian jet MH370 at the daunting depth of 4,500 metres (15,000 feet), abandoning the search for black-box transmissions six days after the last ping was heard.

Angus Houston, the former air marshal who heads the Joint Agency Coordination Centre, revealed also that an oil slick had been sighted in the area of the search led by the Australian navy vessel Ocean Shield far off Perth.

"Ocean Shield will cease searching with the towed pinger locator later today and deploy the autonomous underwater vehicle Bluefin-21 as soon as possible," Houston said, adding it could enter the water Friday evening.

"We haven't had a single detection in six days so I guess it's time to go underwater," he told a news conference in Perth.

Malaysia Airlines Flight MH370 went missing on March 8 en route from Kuala Lumpur to Beijing with 239 people on board, and how the plane may have come to crash in the southern Indian Ocean remains a mystery.

So far no debris has been found despite an enormous search involving ships and planes from several nations. But Houston said about two litres of the newly spotted oil slick had been collected for testing.

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

It would be a number of days before the oil could be conclusively tested ashore, but Houston said he did not think it was from one of the many ships involved in the search.

"It's very close to where the transmissions are coming from and we'll investigate it and that will take a little bit of time given that we're in the middle of the Indian Ocean.

"We don't think it's from the ships, so where is it from? So it's another lead to pursue."

'Slow and painstaking'

Houston emphasised that it was 38 days since the Boeing 777 vanished and the batteries powering the black box tracker beacons had a shelf life of only 30 days.

Ocean Shield has detected four signals linked to aircraft black boxes, helping to narrow down the vast search zone, but the last confirmed ping came on Tuesday last week and officials suspect the batteries are now dead.

The Bluefin-21 is equipped with side-scanning sonar and will initially focus on 40 square kilometres (15 square miles) of sea floor in the vicinity of the detected signals.

But Houston explained that the US-made vessel operates slowly, with each mission taking a minimum of 24 hours to complete.

The device needs two hours to reach the bottom where it will work for 16 hours producing a high-resolution 3D map before surfacing in another two hours.

Downloading and analysing data requires a further four hours.

But while the mini-sub could take the search a step closer towards visually identifying any wreckage, Houston repeated his long-standing note of caution that nothing was guaranteed.

He noted that after Air France Flight AF447 crashed in the Atlantic Ocean in June 2009, it took nearly two years to retrieve the black boxes from a depth of 3,900 metres.

"However, this is the best lead we have and it must be pursued vigorously. Again I emphasise that this will be a slow and painstaking process."

The Bluefin-21, a 4.93-metre long sonar device, weighs 750 kilograms and can operate down to 4,500 metres -- roughly the depth of the ocean floor where the pings were detected.

Houston also said the search for floating material from the plane would be concluded in the coming days.

"The chances of any floating material being recovered have greatly diminished and it will be appropriate to consult with Australia's partners to decide the way ahead later this week," he added.