PERTH (AFP) - The search for missing Malaysia Airlines Flight MH370 will likely continue for "a long time", Australian Prime Minister Tony Abbott warned on Saturday, a day after voicing confidence that signals from the black box had been detected.
He appeared to step back from the most upbeat official assessment so far when he hinted on Friday that a breakthrough was imminent.
When the prime minister announced from Shanghai that he would say no more of his "high confidence" before talking to the Chinese leadership, speculation swirled through the media that a breakthrough was imminent.
Retired air chief marshal Angus Houston, who heads the hunt from Perth, had quickly issued a statement clarifying that there had been no breakthrough.
On Saturday, Mr Abbott repeated his confidence in the search, but put the accent on the difficulties remaining.
"We do have a high degree of confidence that the transmissions we have been picking up are from flight MH370," Mr Abbott said on the last day of his visit to China.
But he added that "no one should under-estimate the difficulties of the task ahead of us".
"Yes we have very considerably narrowed down the search area but trying to locate anything 4.5 kilometres beneath the surface of the ocean about a thousand kilometres from land is a massive, massive task and it is likely to continue for a long time to come."
The Australian-led search for the Boeing 777, which disappeared en route from Kuala Lumpur to Beijing, is racing to gather as many signals as possible to determine an exact resting place before a submersible is sent down to find wreckage.
The Joint Agency Coordination Centre (JACC) said on Saturday that the remote search area where the plane was believed to have gone down some was still shrinking.
"Today, Australian defence vessel Ocean Shield continues more focused sweeps with the towed pinger locator to try and locate further signals related to the aircraft's black boxes," JACC said.
Ocean Shield has picked up four signals linked to aircraft black boxes, with the first two analysed as being consistent with those from aircraft flight recorders.
The beacons on the plane's flight data and cockpit voice recorders have a normal battery lifespan of around 30 days. MH370 vanished on March 8 while travelling from Kuala Lumpur to Beijing with 239 people onboard.
AP-3C Orion surveillance aircraft were also carrying out acoustic searches in conjunction with Ocean Shield, the statement said, adding that the British oceanographic ship HMS Echo was also working in the area.
Saturday's total search zone covers 41,393 sq km and the core of the search zone lies 2,330 km north-west of Perth.
"This work continues in an effort to narrow the underwater search area for when the Autonomous Underwater Vehicle is deployed," JACC said, adding that there have been no confirmed signal detections over the past 24 hours.
Speaking on Friday in China, home to two-thirds of the 239 people on board MH370, Mr Abbott suggested the mystery about the plane's fate might soon be solved.
"We have very much narrowed down the search area and we are very confident the signals are from the black box," Mr Abbott said, although the transmissions were "starting to fade".
"We are confident that we know the position of the black box flight recorder to within some kilometres," he said. He later met Chinese President Xi Jinping at the Great Hall of the People in Beijing.
But Mr Houston struck a much more cautious note just afterwards, saying "there has been no major breakthrough in the search for MH370". He said the Ocean Shield would continue to trawl for pings.
"It is vital to glean as much information as possible while the batteries on the underwater locator beacons may still be active," he said.
A decision to deploy a submersible sonar device "could be some days away", he added.