But at least four orange objects that were more than 2 meters (6 feet) in size were seen by the crew of an Australian P3 Orion search plane, said the pilot, Flight Lt. Russell Adams, after returning to base.
"I must stress that we can't confirm the origin of these objects," he said, adding that images of the items have yet to be verified, and a GPS buoy was dropped and ships must still investigate.
Adams said it was "the most visibility we had of any objects in the water and gave us the most promising leads."
The planes and ships are scouring a search zone that was redefined Friday based on satellite data from the Boeing 777, but they have found no debris associated with the flight, said Australian Navy Commodore Peter Leavy. The zone lies in a shipping lane where sea trash is common, complicating the effort.
Australian Prime Minister Tony Abbott insisted the "intensifying search effort" was positive because objects "have been recovered from the ocean" in the zone after a weeklong search in another area saw items from planes that ships never managed to find.
The planes taking part in Sunday's search included three Australian P3 Orions, a Japanese P3, a Chinese Il-76, a Korean Orion, a U.S. Poseidon, and two Malaysian C-130s. Eight ships were on the scene, an area roughly the size of Poland or New Mexico, about 1,850 kilometers (1,150 miles) west of Australia. The vessels include the Australian navy supply ship HMAS Success, which was designated to carry any wreckage found.
Ten planes and 10 ships will take part in Monday's search, the maritime agency said, with some parts of the zone expected to experience low clouds and rain — similar to Sunday's weather.
"We have got to have conclusive visual evidence of debris," said Cmdr. William Marks, a spokesman for the U.S. Navy's 7th Fleet, appearing on CBS' "Face the Nation." "That is the most important thing. So we have to keep flying these missions out of Perth."
Abbott said a former Australian defense chief, Air Chief Marshal Angus Houston, will head a new center in Perth for search and recovery operations, coordinating the international effort.
Although searchers were hampered by rain and low clouds Sunday, they were still were able to look for debris because of visibility of about 10 kilometers (6 miles). It takes planes about 2 1/2 hours to get to the area, allowing a five-hour search before they must return.
Other objects spotted so far include three with white, red and orange colors by the Chinese plane, China's official Xinhua News Agency said. The missing Boeing 777's exterior was red, white, blue and gray.
For a full week, searchers relied on satellite imagery from various countries as they tried to find the plane in a zone to the south of the current area. They abruptly shifted the site Friday after authorities concluded the plane could not have traveled as far as they had thought based on its estimated speed and fuel consumption.
That prompted the change in the search for the plane's likely entry point into the sea and the possible location of its flight recorders, or "black boxes," which should contain clues to what caused the aircraft to fly so far off-course.
The Ocean Shield, an Australian warship with a U.S. device that detects "pings" from the flight recorders was joining the search. It will still take three to four days for the Ocean Shield to reach the search zone.
"The ship will take part in the surface search until the debris is positively identified and an underwater search area is then predicted," U.S. Navy Capt. Mark Matthews said.