The four planes were checking to see if two large objects spotted in satellite imagery bobbing in the remote ocean were debris from Fight 370 that disappeared March 8 with 239 people on board.
One of the objects was 24 meters (almost 80 feet) in length and the other was 5 meters (15 feet). There could be other objects in the area, a four-hour flight from Australia's southwestern coast, said John Young, manager of the Australian Maritime Safety Authority's emergency response division.
"This is a lead, it's probably the best lead we have right now," Young said. He cautioned that the objects could be seaborne debris along a shipping route where containers can fall off cargo vessels, although the larger object is longer than a container.
A statement from the authority said the four planes searched an area of 23,000 square kilometers (8,800 square miles) about 2,500 kilometers (1,550 miles) southwest of Perth on Thursday without success. The area is about halfway between Australia and desolate islands off the Antarctic.
"The search will continue on Friday," it said. It earlier said the search had been hampered by low visibility caused by clouds and rain.
News that possible plane parts had been found marked a new phase in the emotional roller coaster for distraught relatives of the passengers, who have criticized Malaysia harshly for not releasing timely information about the plane. While they still hope their loved ones will somehow be found, they acknowledged that news of the debris could mean the plane plunged into the ocean.
"If it turns out that it is truly MH370 then we will accept that fate," said Selamat Bin Omar, the father of a Malaysian passenger on the jet, which carried mostly Chinese and Malaysian nationals.
But he cautioned that relatives still "do not yet know for sure whether this is indeed MH370 or something else. Therefore we are still waiting for further notice from the Australian government."
Malaysian Defense Minister Hishammuddin Hussein told a news conference Thursday that "for all the families around the world, the one piece of information that they want most is the information we just don't have - the location of MH370."
Malaysian officials held a meeting Thursday night with the relatives in a hotel near Kuala Lumpur, but journalists were kept away.
After the meeting, groups of people left looking distraught.
Hamid Amran, who had a child on Flight 370, said questions asked at the meeting made it "apparent that Malaysia's military is incapable of protecting its own airspace."
He said he "believes that my child and all the other passengers are still alive. I will not give up hope."
A man who would only give his surname, Lau, said he was there to support a Chinese couple who had lost their only son.
"It appears some families are slowing accepting the worst outcome," he said.
A group of Malaysian government and airline officials also flew to Beijing on Thursday night to meet families there.
Young said the depth of the ocean in the latest area, which is south from where the search had been focused since Monday, is several thousand meters (yards).
He said it may be difficult to spot the objects as they "are relatively indistinct on the imagery ... but those who are experts indicate they are credible sightings. The indication to me is of objects that are a reasonable size and probably awash with water, moving up and down over the surface."
The Australian Maritime Safety Authority released two images of the whitish objects. They were taken March 16, but Australian Air Commodore John McGarry said it took time to analyze them.
"The task of analyzing imagery is quite difficult, it requires drawing down frames and going through frame by frame," he said.
An Australian C-130 Hercules plane dropped marker buoys in the area to aid in the search.
But some analysts said the debris is most likely not pieces of Flight 370. "The chances of it being debris from the airplane are probably small, and the chances of it being debris from other shipping are probably large," said Jason Middleton, an aviation professor at the University of New South Wales in Sydney.
The hunt for the Boeing 777 has been punctuated by several false leads since it disappeared above the Gulf of Thailand. Oil slicks that were spotted did not contain jet fuel. A yellow object thought to be from the plane turned out to be a piece of sea trash. Chinese satellite images showed possible plane debris, but nothing was found.
But this is the first time that possible objects have been spotted since the search area was massively expanded into two corridors, one stretching from northern Thailand into Central Asia and the other from the Strait of Malacca down to southern reaches of the Indian Ocean.
Hishammuddin made clear that although international search efforts are continuing both on land and in sea in the northern and southern hemispheres, the effort is mostly concentrated south of the equator over the vast Indian Ocean.
Out of a total of 29 aircraft, 18 ships and six ship-borne helicopters deployed in the operation, only four aircraft are now scouring the north.
The Norwegian cargo vessel Hoegh St. Petersburg was rerouted and arrived at the area where the possible wreckage was spotted. It used searchlights after dark to scan the seas for any debris.
"They (the ship) have been asked to continue the search tomorrow and they will continue tomorrow morning," Ingar Skiaker from Hoegh Autoliners told a news conference in Oslo.
The Norwegian ship, which transports cars, was on its way from South Africa to Australia, he said. The Australian Maritime Safety Authority said another commercial ship and an Australian navy vessel were also en route to the search area.
Flight 370 disappeared on a night flight from Kuala Lumpur to Beijing. Malaysian authorities have not ruled out any possible explanation, but have said the evidence so far suggests the plane was deliberately turned back across Malaysia to the Strait of Malacca, with its communications systems disabled. They are unsure what happened next.
Police are considering the possibility of hijacking, sabotage, terrorism or issues related to the mental health of the pilots or anyone else on board.
Malaysian authorities have said that files were deleted Feb. 3 from the home flight simulator of the missing plane's pilot, Zaharie Ahmad Shah, and Hishammuddin said he had no new information on efforts to recover those files.
The FBI has joined forces with Malaysian authorities in analyzing deleted data on the simulator. It was not clear whether investigators thought that deleting the files was unusual. They might hold hints of unusual flight paths that could help explain where the missing plane went, or the files could have been deleted simply to clear memory for other material.
Gelineau reported from Sydney, Australia. Associated Press writers Rod McGuirk and Todd Pitman in Kuala Lumpur, Nick Perry in Wellington, New Zealand, and Julia Gronnevet in Oslo, Norway, contributed to this report.