Malaysia Airlines flight MH370: Analysis of Indian Ocean oil slick shows it is not from missing jet

Submersible vehicle Bluefin 21 returns from first full mission - but nothing of note found yet

The authorities looking for Malaysia Airlines flight MH370 have said that tests on an oil slick found near the search area show it didn't come from the missing Boeing 777.

Experts analysed samples of oil taken from the slick at a lab in the western Australian city of Perth, but they tested negative for either aircraft fuel or hydraulic liquid.

It had been hoped that the oil would provide further evidence that crews are looking in the right place for the jet, after it was spotted about 5.5km (3.4 miles) from the area where a vessel picked up signals consistent with those given off by an aircraft's "black box" flight recorders.

Those sounds allowed recovery teams to identify a narrow stretch of the sea bed in the southern Indian Ocean where it is believed the plane's wreckage came to rest.

It is currently being scoured by the unmanned submersible vehicle Bluefin 21, which on Thursday finally managed to complete a full 16-hour mission to the silt-covered ocean floor.

The vessel's two previous underwater ventures were called short, one because waters became too deep and the other because of a technical issue.

The centre orchestrating the search said that data from the third search was still being analysed at the end of another day, and said that nothing of note had yet been discovered. The sub has now covered 90 square kilometres (35 square miles) of sea floor.

A search team consisting of 12 planes and 11 ships were still scouring a 40,300-square-km (15,600-square-mile) patch of sea for any debris that may be floating on the ocean surface today, about 2,200 km (1,400 miles) northwest of Perth.

Despite weeks of searching, no debris related to the jet has been found and earlier this week, search effort leader Angus Houston said the surface hunt would be ending within days. But the search coordination centre on Thursday said crews would continue searching the ocean surface into next week.

Malaysia's defense minister, Hishamuddin Hussein, confirmed that the search would continue through 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 at a news conference Thursday.

Radar and satellite data show the Boeing 777 flew far off-course on 8 March for an unknown reason and would have run out of fuel in a desolate patch of the Indian Ocean west of Australia.

The US Navy's unmanned sub cut short its first mission on Monday because it exceeded its maximum operating depth of 4,500 metres (15,000 feet). 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 today 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.

In addition to finding the plane itself, investigators want to recover the black boxes in hopes the cockpit voice and flight data recorders can explain why the plane lost communications and flew so far off-course before disappearing.

Additional reporting by AP