Australian officials pledged to continue the search for two large objects spotted by a satellite earlier this week, which had raised hopes that the two-week hunt for the Boeing 777 that disappeared March 8 with 239 people on board was nearing a breakthrough.
But Australia's acting prime minister, Warren Truss, tamped down expectations.
"Something that was floating on the sea that long ago may no longer be floating — it may have slipped to the bottom," he said. "It's also certain that any debris or other material would have moved a significant distance over that time, potentially hundreds of kilometers."
In Kuala Lumpur, where the plane took off for Beijing, the country's defense minister thanked more than two dozen countries involved in the search that is stretching from Kazakhstan in Central Asia to the southern Indian Ocean, and said the focus remains on finding the airplane.
"This going to be a long haul," Hishammuddin Hussein told a news conference.
The search area indicated by the satellite images — some 2,500 kilometers (1,550 miles) southwest of Perth — is so remote it takes aircraft four hours to fly there and four hours back, leaving them with only enough fuel to search for about two hours.
On Friday, five planes, including three P-3 Orions, made the trip. While search conditions had improved from Thursday, with much better visibility, the Australian Maritime Safety Authority said there were no sightings of plane debris.
Searchers relied mostly on trained spotters aboard the planes scanning the ocean rather than radar because the use of radar found nothing during the first day of the search on Thursday, Australian officials said.
Going forward, the search will focus more on visual sightings because civilian aircraft are being brought in to participate. The military planes will continue to use both radar and spotters.
"Noting that we got no radar detections yesterday, we have replanned the search to be visual. So aircraft flying relatively low, very highly skilled and trained observers looking out of the aircraft windows and looking to see objects," said John Young, manager of the maritime safety authority's emergency response division.
Two Chinese aircraft are expected to arrive in Perth on Saturday to join the search, and two Japanese aircraft will be arriving Sunday, Truss said. A small flotilla of ships coming to Australia from China was still several days away.
"We are doing all that we can, devoting all the resources we can and we will not give up until all of the options have been exhausted," said Truss, who is acting prime minister while Tony Abbott is in Papua New Guinea.
Experts say it is impossible to tell if the grainy satellite images of the two objects — one 24 meters (almost 80 feet) long and the other measuring 5 meters (15 feet) — were debris from the plane. But officials have called this the best lead so far in the search that began March 8 after the plane vanished over the Gulf of Thailand on an overnight flight to Beijing.
For relatives of the people aboard the plane — 154 of the 227 passengers are Chinese — hope was slipping away, said Nan Jinyan, sister-in-law of passenger Yan Ling.
"I'm psychologically prepared for the worst and I know the chances of them coming back alive are extremely small," said Nan, one of dozens of relatives gathered at a Beijing hotel awaiting any word about their loved ones.
Abbott spoke with Chinese President Xi Jinping, whom he described as "devastated."
"It's about the most inaccessible spot that you could imagine on the face of the Earth, but if there is anything down there we will find it. We owe it to the families of those people to do no less," Abbott said.
The Norwegian cargo vessel Hoegh St. Petersburg is also in the area helping with the search. The ship, which transports cars, was on its way from South Africa to Australia.
Haakon Svane, a spokesman for the Norwegian Shipowners' Association, said the ship and its crew of 20 Filipinos had searched a strip of ocean stretching about 100 nautical miles (115 miles; 185 kilometers) using binoculars and unaided eyes.
"The visual observations are the most important. The fact that they are there and have the capacity to move in a specific pattern is the most important contribution," he said.
The Australian Maritime Safety Authority said another commercial ship was also in the area and an Australian navy vessel was en route, and AMSA officials also were checking to see if there was any new satellite imagery that could provide searchers with more information.
Pieces of an aircraft have been found floating for days after a crash into the ocean. Peter Marosszeky, an aviation expert at the University of New South Wales, said the plane's wing could remain buoyant for weeks if the fuel tanks inside were empty and had not filled up with water.
Other experts said that if the aircraft breaks into pieces, normally only items such as seats and luggage would remain floating.
"We seldom see big metal (pieces) floating. You need a lot of (buoyant) material underneath the metal to keep it up," said Lau Kin-tak, an expert in aircraft maintenance and accidents at Hong Kong Polytechnic University.
The anguished relatives of passengers met Friday with Malaysian officials at the Beijing hotel. Attendees said they had a two-hour briefing about the search but that nothing new was said.
Wang Zhen, son of missing artist Wang Linshi, said there were questions about why Malaysian authorities had provided so much seemingly contradictory information.
Wang said he still has hopes his father can be found alive and is praying that the satellite sightings turn out to be false. He said he and other relatives are suspicious about what they are being told by the Malaysian side, but are at a loss as to what to do next.
"We feel they're hiding something from us," said Wang, who is filling his days attending briefings and watching the news for updates.
Malaysian authorities have not ruled out any possible explanation for what happened to the jet, but have said the evidence so far suggests it 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 possibilities of hijacking, sabotage, terrorism or issues related to the mental health of the pilots or anyone else on board.
Gelineau reported from Sydney, Australia. Associated Press writers Todd Pitman and Scott McDonald in Kuala Lumpur, Malaysia; Nick Perry in Wellington, New Zealand; Kelvin Chan in Hong Kong; Christopher Bodeen and Isolda Morillo in Beijing; and Mark Lewis in Stavanger, Norway, contributed to this report.