Investigators who analysed samples of an oil slick in the hunt for the missing Malaysian airliner say that it did not come from the plane.
The search co-ordination centre said the oil tested in the western Australian city of Perth came up negative for aircraft oil or hydraulic fluid. The oil was collected earlier this week from a slick about 5.5 kilometres from the area where equipment picked up underwater sounds consistent with an aircraft black box.
It had been hoped that the oil would be evidence that officials are looking in the right place for flight 370, which vanished March 8 while en route from Kuala Lumpur to Beijing with 239 passengers and crew on board. Searchers have yet to find any physical proof that the sounds that led them to the ocean floor were from the ill-fated jet.
The centre has begun analysing data collected by a robotic submarine that completed its first successful scan of the seabed yesterday.
The unmanned sub's first two missions were cut short by technical problems and deep water, but the Bluefin 21 finally managed to complete a full 16-hour scan of the silt-covered seabed far off Australia's west coast.
While data collected during the mission was still being analysed, nothing of note had been discovered, the centre said. There are fears that wreckage could have been enveloped in a thick layer of mud-like silt on the ocean floor, complicating detection and eventual recovery.
The US Navy's unmanned submarine cut short its first mission on Monday because it exceeded its maximum operating depth of 4,500 metres. Searchers moved it away from the deepest waters before redeploying the sub to scan the seabed with sonar to map a potential debris field.
But the centre said yesterday that officials are now confident that the sub can safely go deeper than was thought, allowing it to cover the entire search area, which has been narrowed based on further analysis of the four underwater signals.
Earlier this week, the search effort leader, Angus Houston, had said the surface hunt would be ending within days. But the search coordination centre said crews would continue searching the ocean surface into next week. Malaysia's defence minister, Hishamuddin Hussein, confirmed that the search would continue through the Easter weekend, but acknowledged that officials would have to rethink their strategy at some point if nothing is found.
"There will come a time when we need to regroup and reconsider, but in any event, the search will always continue. It's just a matter of approach," he said.
Radar and satellite data show the Boeing 777 flew far off course for an unknown reason and would have run out of fuel over a desolate patch of the Indian Ocean west of Australia.