Search teams are preparing to use a mini-submarine to scan the sea bed for wreckage from flight MH370 "as soon as possible".

Air Chief Marshal Angus Houston, leading the search, told a news conference an oil slick had been found within the current search zone of the southern Indian Ocean.

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

Mr. Houston cautioned that the use of unmanned submarine, Bluefin-21, should not raise hopes that debris from the aircraft will be found.

"Again, I emphasise that this will be a slow process" he said.

He said two litres of oil were found by Australian vessel Ocean Shield in the area where four "pings" possibly from a black box recorder were detected last week.

He has given this update this morning.

The oil is being examined to see if it is aviation fuel, and that process could take several days. The slick was found 5,500 metres downwind and down sea from where the possible signals were detected.

11 military aircraft, one civil aircraft and 15 ships are scouring an area of more than 47,600 square kms in today's search.

The centre of the search zone is around 2,200kms northwest of Perth on the western coast of Australia. Ocean Shield will stop using its Towed Pinger Locator to try to locate the black boxes of the Boeing 777 later today.

The submarine will then be deployed from the vessel. Each of its missions will take 24 hours and the first will cover an area 5km by 8km, Mr. Houston said.

The US-manufactured submarine, a 16.2 feet (4.93m) long sonar device, can operate at a depth of up to 14,700 feet (4,500m), roughly the depth of the ocean floor where the "pings" were detected.

There are fears the plane's black boxes have now stopped transmitting signals, as the batteries last around a month and the plane disappeared more than five weeks ago.

Bad weather is expected to hit the search area this week, making the search more difficult. The jet, which was carrying 239 people, vanished while flying from Kuala Lumpur to Beijing on March 8th.