PERTH, Australia — With no new underwater signals detected, Australian Prime Minister Tony Abbott said Saturday that the massive search for the missing Malaysian jet would likely continue “for a long time,” with electronic transmissions from the plane’s black boxes fading fast.

Abbott appeared to couch his comments from a day earlier while on a visit to China, where he met with President Xi Jinping. He 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 Malaysia Airlines Boeing 777’s black boxes.

He continued to express this belief Saturday, but added that the job of finding the plane, which disappeared March 8 en route from Kuala Lumpur, Malaysia, to Beijing, 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 370.

“No one should underestimate the difficulties of the task still ahead of us,” he said on the last day of his China trip.

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

After analyzing satellite data, officials believe the plane — which was carrying 239 people, including 153 Chinese passengers — flew off course for an unknown reason and went down in the southern Indian Ocean off the west coast of Australia.

Search crews are scrambling because the batteries powering the 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,” Abbott said. “So that we can narrow the search area down to as small an area as possible.”

The underwater search zone is currently a 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. That’s about six weeks to two months to canvass the current underwater zone. The signals are also coming from 15,000 feet below the surface, which is the deepest the Bluefin can dive. The search coordination center has said it was considering options in case a deeper-diving sub is needed.

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