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PERTH, Australia (AP) A day after expressing optimism about the hunt for the missing Malaysian jet, Australia's leader warned Saturday that the massive search would likely continue "for a long time."

"No one should underestimate the difficulties of the task still ahead of us," Prime Minister Tony Abbott said in Beijing, on the last day of his China trip.

Abbott appeared to couch his comments from a day earlier, when he met in Beijing with Chinese President Xi Jinping to brief him on the search for Malaysia Airlines Flight MH370, which was carrying 239 people most of them Chinese when it disappeared March 8 en route from Kuala Lumpur to Beijing.

After analyzing satellite data, officials believe the plane flew off course for an unknown reason and went down in the southern Indian Ocean off Australia's west coast.

Abbott said Friday that he was "very confident" signals heard by an Australian ship towing a U.S. Navy device that detects flight recorder pings were coming from the missing Boeing 777's black boxes.

He continued to express this belief Saturday, but with no new underwater signals detected in the past few days and electronic transmissions from the black boxes fading fast, Abbott said the job of finding the plane remained arduous. Recovering the plane's flight data and cockpit voice recorders is essential for investigators to try to piece together what happened to Flight MH370.

We have "very considerably narrowed down the search area, but trying to locate anything 4.5 kilometers (2.8 miles) beneath the surface of the ocean about 1,000 kilometers (620 miles) from land is a massive, massive task, and it is likely to continue for a long time to come," Abbott said.

"There's still a lot more work to be done and I don't want anyone to think that we are certain of success, or that success, should it come, is going to happen in the next week or even month. There's a lot of difficulty and a lot of uncertainty left in this," he said.

In Malaysia, Defense Minister Hishammuddin Hussein on Saturday refuted a front-page report in a local newspaper, the New Strait Times, that a signal from the mobile phone of co-pilot Fariq Abdul Hamid was picked up by a telecommunications tower near the Malaysian city of Penang shortly before the plane disappeared from radar. The newspaper report said the signal ended abruptly before contact was established.

Hishammuddin, who is also the acting transport minister, told the Malaysian national news agency Bernama that he should have been aware of the phone call earlier, but that wasn't the case.

"I cannot comment (on the newspaper report) because if it is true, we would have known about it much earlier," Hishammuddin said after praying at a mosque in southern Jofor state, according to Bernama.

He added that it was irresponsible for anyone to take the opportunity to make "baseless" reports.

In the southern Indian Ocean, search crews are scrambling because the batteries powering the flight recorders' locator beacons last only about a month, and that window has already passed. Finding the devices after the batteries die will be extremely difficult due to the extreme depth of the water in the search area.

Two sounds heard a week ago by the Australian ship Ocean Shield, which was towing the ping locator, were determined to be consistent with the signals emitted from the two black boxes. Two more pings were detected in the same general area Tuesday.

"Given that the signal from the black box is rapidly fading, what we are now doing is trying to get as many detections as we can so that we can narrow the search area down to as small an area as possible," Abbott said.

The underwater search zone is currently a 1,300-square-kilometer (500-square-mile) patch of the seabed, about the size of Los Angeles.

The searchers want to pinpoint the exact location of the source of the sounds or as close as they can get and then send down a robotic submersible to look for wreckage. But the sub will not be deployed until officials are confident that no other electronic signals are present.

The Bluefin 21 submersible takes six times longer to cover the same area as the ping locator, and will need about six weeks to two months to canvass the current underwater zone. The signals are also coming from 4,500 meters (15,000 feet) below the surface, which is the deepest the Bluefin can dive. The search coordination center has said it is considering options in case a deeper-diving sub is needed.

The surface area to be searched for floating debris has been narrowed to 41,393 square kilometers (15,982 square miles) of ocean extending from about 2,300 kilometers (1,400 miles) northwest of Perth. Up to 10 planes and 14 ships were searching Saturday.

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Follow Margie Mason on Twitter: twitter.com/MargieMasonAP