Following four strong underwater signals in the past week, all has gone quiet in the hunt for the missing Malaysia Airlines jet, meaning the batteries in the plane's all-important black boxes may finally have died.

Despite having no new pings to go on, crews are continuing their search Sunday for debris and any sounds that could still be emanating. They are desperately trying to pinpoint where the Boeing 777 — that disappeared March 8 en route from Kuala Lumpur to Beijing — could be amid an enormous patch of deep ocean.

No new electronic pings have been heard since April 8, and the batteries powering the locator beacons on the jet's black box recorders may already be dead. They only last about a month, and that window has passed.

Once officials are confident no more sounds will be heard, a robotic submersible will be sent down. Finding the black boxes after the batteries fail will be extremely difficult because the water in the area is 4,500 metres deep.

"We're now into Day 37 of this tragedy," said aviation expert Geoffrey Thomas.

"The battery life on the beacons is supposed to last 30 days. We're hoping it might last 40 days. However, it's been four or five days since the last strong pings. What they're hoping for is to get one more, maybe two more pings so they can do a triangulation of the sounds and try and narrow the [search] area."

The underwater search zone is currently a 1,300-square-kilometre patch of the seabed, about the size of the city of Los Angeles.

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 black boxes. Two more pings were detected in the same general area Tuesday, but no new ones have been picked up since then.

Australian Prime Minister Tony Abbott has expressed confidence that the pings picked up by the Ocean Shield were coming from the plane's two black boxes, but he cautioned that finding the actual aircraft could take a long time.

"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," Abbott said Saturday in Beijing, where he was wrapping up a visit to China.

After analyzing satellite data, officials believe the plane with 239 people aboard flew off course for an unknown reason and went down in the southern Indian Ocean off the west coast of Australia.

Separately, a Malaysian government official said on Thursday that investigators have concluded the pilot spoke the last words to air traffic control, "Good night, Malaysian three-seven-zero," and that his voice had no signs of duress. A re-examination of the last communication from the cockpit was initiated after authorities last week reversed their initial statement that the co-pilot was speaking different words.

The senior government official spoke on condition of anonymity because he wasn't authorized to speak to the media.

The surface area being searched on Sunday for floating debris was 57,506 square kilometres of ocean extending about 2,200 kilometres northwest of Perth. Up to 12 planes and 14 ships were participating in the hunt.

MH370 search map

MH370 search map (CBC News)