* Australian PM "confident" signals are from black boxes
* Search area refined with help of new "pings"
* Sophisticated instruments focusing on refined area
* Officials optimistic that plane could soon be found (Updates with fresh Aust PM quotes, search chief comments)
By Swati Pandey and John Ruwitch
PERTH/SHANGHAI, April 11 (Reuters) - Search and rescue officials in Australia are confident they know the approximate position of the black box recorders from missing Malaysia Airlines Flight MH370, Australian Prime Minister Tony Abbott said on Friday.
At the same time, however, the head of the agency coordinating the search said that the latest "ping" signal, which was captured by a listening device buoy on Thursday, was not related to the plane.
"We are confident that we know the position of the black box flight recorder to within some kilometres (miles)," Abbott said in a speech in the Chinese commercial capital Shanghai.
"Still, confidence in the approximate position of the black box is not the same as recovering wreckage from almost four and a half kilometres beneath the sea or finally determining all that happened on the flight."
The mystery of Malaysia Airlines Flight MH370, which disappeared more than a month ago, has sparked the most expensive search and rescue operation in aviation history.
The search was focusing on a small patch of the Indian Ocean on Friday, after the latest "ping" seemed to lend credence to four previous "pings" detected by a U.S. Navy "Towed Pinger Locator" (TPL) towed by Australia's Ocean Shield vessel.
All five acoustic signals were detected in this small area.
But Angus Houston, head of the Australian agency supervising the search effort, said on Friday that analysis of acoustic data confirmed that the latest signal was unlikely to be related to the missing plane's black boxes.
"On the information I have available to me, there has been no major breakthrough in the search for MH370. I will provide a further update if, and when, further information becomes available," he said in a statement.
The black boxes record cockpit data and may provide answers about what happened to the plane, which was carrying 227 passengers and 12 crew when it vanished on March 8 and flew thousands of kilometres off its Kuala Lumpur-to-Beijing route.
BATTERIES FADING AS SEARCH CLOSES IN
Search efforts are now focused on three areas.
Aircraft and ships are combing two large search zones, some 2,390 km (1,485 miles) northwest of Perth, for possible floating debris related to the crash.
But it is the much smaller search zone, just 600 sq km (232 sq miles, located about 1,670 km (1038 miles) northwest of Perth that has generated fresh optimism.
The smaller zone is near where the Ocean Shield picked up the acoustic signals and where dozens of sonobuoys capable of transmitting data to search aircraft via radio signals were dropped on Wednesday.
The batteries in the black boxes have already reached the end of their 30-day expected life, making efforts to swiftly locate them on the murky ocean floor all the more critical, Abbott said.
"We are now getting to the stage where the signal from what we are very confident is the black box is starting to fade and we are hoping to get as much information as we can before the signal finally expires," he said.
But experts say the process of teasing out the signals from the cacophony of background noise in the sea is a slow and exhausting process.
An autonomous underwater vehicle named Bluefin-21 is onboard the Ocean Shield and could be deployed to look for wreckage on the sea floor once a final search area has been identified.
(Additional reporting by Matt Siegel and Lincoln Feast in SYDNEY; Editing by Michael Perry)
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