PERTH, Australia (AP) -- Aircraft and ships from China headed to the desolate southern Indian Ocean to join the search for Malaysia Airlines Flight 370, now lost for two full weeks, and Australia promised its best efforts to resolve "an extraordinary riddle."
A satellite spotted two large objects in the area earlier this week, raising hopes of finding the Boeing 777 that disappeared March 8 with 239 people on board. Surveillance planes scoured the region — about 2,500 kilometers (1,550 miles) southwest of Perth —for a second day on Friday but came back empty-handed after a 10-hour mission.
Australian officials pledged to continue the effort, even as they tried to tamp down expectations.
"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," Prime Minister Tony Abbott said at a news conference in Papua New Guinea.
"We owe it to the families and the friends and the loved ones of the almost 240 people on Flight MH370 to do everything we can to try to resolve what is as yet an extraordinary riddle," he added.
Two Chinese aircraft are expected to arrive in Perth on Saturday to join the search, and two Japanese aircraft will arrive Sunday. A small flotilla of ships from China is still several days away.
Abbott spoke with Chinese President Xi Jinping, describing him as "devastated." The passengers included 154 Chinese.
In Kuala Lumpur, where the plane took off for Beijing, Malaysian Defense Minister Hishammuddin Hussein thanked the more than two dozen countries involved in the overall search that stretches from Kazakhstan in Central Asia to the southern Indian Ocean. He called the whole process "a long haul."
The search area indicated by the satellite images in the southern Indian Ocean is a four-hour round-trip flight from western Australia, leaving planes with only enough fuel to search for about two hours. The images were taken March 16, but the search in the area did not start until Thursday because it took time to analyze them.
Five planes, including three P-3 Orions, made the trip Friday. While search conditions had improved from a day earlier, 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 rather than radar, which found nothing Thursday, Australian officials said. The search will focus more on visual sightings because civilian aircraft are being brought in. 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.
Malaysia asked the U.S. for undersea surveillance equipment to help in the search, said Rear Adm. John Kirby, a Pentagon spokesman. Defense Secretary Chuck Hagel promised to assess the availability of the technology and its usefulness in the search, Kirby said.
The Pentagon says it has spent $2.5 million to operate ships and aircraft in the search and has budgeted another $1.5 million for the efforts.
There is a limited battery life for the beacons in the cockpit-voice and flight-data recorders — about 30 days, said Chuck Schofield, vice president of business development for Dukane Seacom Inc. He said it's "very likely" that his company made the beacons on the missing jet.
The devices work to a depth of 20,000 feet, with a signal range of about two nautical miles, depending on variables like sea conditions. The signals are located using a device operated on the surface of the water or towed to a depth.
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 those aboard the plane, 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.
The Norwegian cargo vessel Hoegh St. Petersburg is also in the area helping with the search. Haakon Svane, a spokesman for the Norwegian Shipowners' Association, said the ship had searched a strip of ocean stretching about 100 nautical miles (115 miles; 185 kilometers).
The Australian Maritime Safety Authority said another commercial ship also was in the area, and an Australian navy vessel was en route. AMSA officials also were checking to see if there was any new satellite imagery that could provide more information.
Aircraft pieces have sometimes been found floating for days after a sea crash. Peter Marosszeky, an aviation expert at the University of New South Wales, said the wing could remain buoyant for weeks if fuel tanks inside it were empty and had not filled 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.
Flight 370 relatives 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; Hohlbrook Mohr in Jackson, Miss.; Pauline Jelinek in Washington, and Mark Lewis in Stavanger, Norway, contributed to this report.