The Bluefin-21 - an almost 5m-long underwater autonomous vehicle that can create a sonar map of the sea floor - will search for wreckage in an area defined by four signals heard last week.

Officials believe those signals - picked up by the pinger locator towed by an Australian vessel - are consistent with flight recorders.

"Analysis of the four signals has allowed the provisional definition of a reduced and manageable search area on the ocean floor," ACM Houston said.

"The experts have therefore determined that the Australian Defence Vessel Ocean Shield will cease searching with the towed pinger locator later today and deploy the... Bluefin-21 as soon as possible.''

He warned that the submersible search would be a long, "painstaking" process that might, in the end, yield no results.

Each Bluefin-21 mission will last 24 hours, with 16 hours spent on the ocean floor, four hours' diving and resurfacing time, and four hours to download data.

The Ocean Shield had also spotted an oil slick in the same area where the signals had been heard, ACM Houston said, and a sample was being sent for testing.

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

Australian officials have said previously that they are confident they are searching in the right area for the missing plane.

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