Leaders of the two countries heading multinational efforts to solve the mystery of Malaysia Airlines Flight 370 pledged Thursday that no effort would be spared to give the families of those on board the answers they need.

Malaysian Prime Minister Najib Razak flew to Australia for briefings on the search for the missing plane and talks with his Australian counterpart, Tony Abbott, whose country is overseeing the hunt in a huge and remote patch of the Indian Ocean.

“It is a very difficult search — the most difficult in human history. But as far as Australia is concerned, we are throwing everything we have at it,” Abbott said in a media appearance with Najib.

No trace of the Boeing 777 has been found nearly four weeks after it vanished in the early hours of March 8 on a flight from Kuala Lumpur to Beijing with 239 people on board.

Ten planes and nine ships were involved in search operations Thursday, scouring the ocean far off Australia’s southwest corner where investigators believe the plane may have ended up after unknown events occurred on board.

Najib, whose government has been harshly criticized by some victims’ families for giving sometimes conflicting information about the flight and for the slow pace of the investigation, said everyone involved in the search is thinking of the families of victims who are waiting desperately for news.

“I know that until we find the plane, many families cannot start to grieve,” Najib said. “I cannot imagine what they are going through. But I can promise them that we will not give up.

“We want to provide comfort to the families and we will not rest until answers are indeed found. In due time, we will provide a closure for this event,” he said.

Najib met with Abbott at the Australian base near Perth that is serving as the hub for the multinational search effort. They were briefed by Angus Houston, the head of a joint agency overseeing the search.

Although Australia is coordinating the ocean search, the investigation into the tragedy ultimately remains Malaysia’s responsibility. Najib said Australia had agreed to be an “accredited representative in the investigation,” and would work with Malaysia on a comprehensive agreement on the search.

On Wednesday, officials warned the investigation may never fully answer why the airliner disappeared. A dearth of information has plagued investigators from the moment the plane’s transponders, which make the plane visible to commercial radar, were shut off.

Military radar picked up the jet just under an hour later, way off course on the other side of the Malay Peninsula. Authorities say that until then, its “movements were consistent with deliberate action by someone on the plane,” but have not ruled out anything, including mechanical error.

Police are investigating the pilots and crew for any evidence suggesting they may have hijacked or sabotaged the plane. The backgrounds of the passengers have been checked by investigators and nothing suspicious has been found.

The search for the plane began over the Gulf of Thailand and South China Sea, where its last voice communications were, and then shifted west to the Strait of Malacca. Experts then analyzed hourly satellite “handshakes” between the plane and a satellite and now believe it crashed somewhere in the southern Indian Ocean.

Thursday’s search zone was a 223,000-square kilometer (86,000-square mile) patch of ocean 1,680 kilometers (1,040 miles) northwest of Perth, part of a larger area crews have been scouring since last week.

The British navy’s HMS Echo reported one alert as it searched for sonic transmissions from the missing plane’s flight data recorder, but it was quickly discounted as a false alarm, the Joint Agency Coordination Center overseeing the search said Thursday.

False alerts can come from animals such as whales, or interference from shipping noise.

No confirmed trace of the plane’s wreckage has been found. Houston has said there is no timeframe for ending the search, but acknowledged a new approach will eventually be needed if nothing turns up.

Australia’s prime minister said everything that possibly could be done to find the plane would be done, but cautioned, “We cannot be certain of success.”

Najib’s wife, Rosmah Mansor, also traveled to Perth, where she met with Danica Weeks, whose husband, Paul Weeks, was among those on Flight 370. Weeks said the meeting gave her some comfort and confidence the Malaysians are committed to finding answers. But she also said the pervasive uncertainty surrounding the plane’s fate had made coping with the loss impossible.

“You cannot grieve for someone unless you have something concrete,” Weeks told Australia’s Channel 9.

Two British vessels — a nuclear-powered submarine with advanced underwater search capability and the British Survey ship HMS Echo — have joined the hunt, Houston said. The Ocean Shield, an Australian warship carrying a U.S. device that detects “pings” from the plane’s flight recorders, was en route.

Spotting wreckage is key to narrowing the search area and ultimately finding the plane’s data recorders, which would provide a wealth of information about the condition the plane was flying under and the communications or sounds in the cockpit.

The data recorders emit a ping that can be detected by special equipment in the immediate vicinity. But the battery-powered devices stop transmitting the pings about 30 days after a crash. Locating the data recorders and wreckage after that is possible, but becomes an even more daunting task.

(AP)