By Ross Kelly in Perth, David Winning in Sydney
Wall Street Journal
April 3, 2014

The Australian head of the international search for Malaysia Airlines Flight 370 has called the operation one of the “most challenging” he has ever seen.

As the chances dim for finding Malaysia Airlines 370’s “black box” flight recorders before the batteries in their locator beacons run out, Malaysian and Australian leaders sought to inject new momentum into a search of the southern Indian Ocean that has yet to find plane wreckage.

Malaysian Prime Minister Najib Razak met aircrews involved in the multinational search for Flight 370 at Pearce air base, near Perth, on Thursday. The visit is Mr. Najib’s first to Australia since the focus of the search swung abruptly to the southern Indian Ocean on March 20, based on satellite images of possible plane debris. So far, nothing related to the missing plane has been found.

Malaysian investigators still believe Flight 370 crashed in the ocean when it ran out of fuel—thousands of kilometers from the nearest airport—hours after disappearing from civilian radar on March 8 with 239 passengers and crew on board.

“The search area is vast and conditions are not easy, but the new refined search area has given us new hope,” Mr. Najib told reporters.

But with uncertainty persisting about whether it is the right area, investigators from five countries, including the U.S. and China, are continuing to analyze satellite and communications data that shed light on how the plane was traveling, including its speed and altitude.

Australian Prime Minister Tony Abbott on Thursday described the search as possibly the most difficult in human history and reiterated that there was no certainty of a breakthrough.

“On the basis of just small pieces of information, we are putting the jigsaw together,” he said. “Every day we have a higher degree of confidence that we know more about what happened to this ill-fated flight.”

On Thursday, 10 aircraft and nine ships were dispatched to the search area, which lies some 1,680 kilometers (about 1,000 miles) northwest of Perth, to look for wreckage and drop marker buoys that can help authorities understand the impact of ocean currents in dispersing any plane debris. The area largely overlaps the one scoured fruitlessly by aircrews and ships the previous day.

Authorities said late Thursday that no new objects had been spotted.

Now in its fourth week, the hunt for Flight 370 has yielded only unrelated scraps of junk. The current search area is in a part of the ocean where currents frequently bring together floating garbage.

This week, the search was joined by a U.K. military submarine equipped to detect signals from the flight-data and cockpit-voice recorders, whose beacons could run out of power as soon as this weekend. The HMS Tireless bolsters the capabilities of the multinational team, which has been relying on a combination of satellite images, radar data and crews scanning through aircraft windows to search an area the size of Italy for floating debris.

The nuclear-powered submarine, built for the Royal Navy as a Cold War attack vehicle, has equipment on board that may help it to pinpoint signals from Flight 370’s recorders. It could also be used to search for aircraft wreckage along the largely undisturbed seabed, a spokeswoman for the U.K.’s defense ministry said.

Peter Jennings, a defense expert at the Australian Strategic Policy Institute, said submarines are of limited use in deep-water searches, though. Most are designed to operate in less than a kilometer (0.6 miles) of water. Depths in the current search area are estimated at up to four times that.

The best hope, he said, lies with sonar systems towed by ships, such as a U.S. Navy black-box locator that left Perth aboard the Australian navy vessel Ocean Shield Monday night.

But Ocean Shield, built to operate in Antarctic weather, isn’t expected to arrive in the search area before April 5—as little as two days the locator-beacon batteries, which have an estimated life of 30 days, expire.

—Rachel Pannett in Sydney contributed to this article.