PERTH, Australia -- 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. Three Australian planes took off at dawn Saturday for a third day of scouring the region about 1,550 miles southwest of Perth.

Australian officials tried to tamp down expectations after a fruitless search Friday, even as they pledged to continue the effort.

"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.

A total of six aircraft were to search the region Saturday: two ultra long-range commercial jets and four P3 Orions, the Australian Maritime Safety Authority said.

While the Orions are only able to search for two hours at a time, the commercial jets can stay in the search area for five hours before they must head back to the base.

Two merchant ships were in the area and the HMAS Success, a navy supply ship, was due to arrive late Saturday afternoon. Weather in the search zone was expected to be relatively good, with some cloud cover.

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.

AMSA officials also were checking to see if there was any new satellite imagery that could provide more information.

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."

Searchers on Friday 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.

Malaysia asked the U.S. for undersea surveillance equipment to help in the search, said Rear Adm. John Kirby. Defense Secretary Chuck Hagel promised to assess the availability of the technology and its usefulness in the search, Kirby said.