Oil slick detected last Sunday by the Australian defense vessel Ocean Shield in the Southern Indian Ocean, where a search is going on for a missing Malaysian airliner was not aircraft engine oil or hydraulic fluid.

The Joint Agency Coordination Centre (JACC), which is overseeing the search operation for Malaysia Airlines Flight MH370, said the result was confirmed based on the preliminary analysis done on the sample collected which arrived in Perth this morning.

On Monday, JACC chief coordinator Air Chief Marshal (Rtd) Angus Houston told a press conference that the oil slick was detected about 5,500m within the vicinity of 'pings' previously detected by the towed pinger locator used by Ocean Shield.

Two litres of the oil slick were collected as sample.

On another development, JACC said Phoenix International, with the assistance of Bluefin Robotics, had assessed that there was a small but acceptable level of risk in operating the Autonomous Underwater Vehicle (AUV) called Bluefin-21, in depths in excess of 4,500 metres.

"This expansion of the operating parameters allows the Bluefin-21 to search the sea floor within the predicted limits of the current search area," the agency said in a statement.

United States-based Phoenix International is the owner of the Bluefin-21, and Bluefin Robotics, its maker.

The JACC also denied reports by some media before this which stated that it would take Bluefin-21 between six weeks and two months to scan the entire underwater search area, saying the statement was incorrect.

Bluefin-21, a probe equipped with side-scan sonar that uses acoustic sounds to create a 3D map of the sea floor, was deployed underwater three times since its maiden mission on Monday. However, data collected from its previous two missions found nothing so far.

It was dispatched in the hope of locating any debris of the missing plane underwater, since no further confirmed signals were picked up by the towed pinger locator since April 8.

Flight MH370, with 239 people onboard, left the KL International Airport at 12.41am on March 8 and disappeared from radar screens about an hour later while over the South China Sea. It was to have arrived in Beijing at 6.30am the same day.

A multinational search was mounted for the Boeing 777-200 aircraft, first in the South China Sea and then, after it was learnt that the plane had veered off course, in the southern Indian Ocean.

After an analysis of satellite data indicated that the plane's last position was in the middle of the Indian Ocean, west of Perth, Australia, Malaysian Prime Minister Datuk Seri Najib Tun Razak announced on March 24 that Flight MH370 "ended in the southern Indian Ocean".