More nations have joined the search in the southern Indian Ocean for Malaysia Airlines flight MH370, amid concern that rain will hamper search efforts.
Ten planes were set to scour the area for possible debris picked up earlier by radar echoes and satellite imagery.
Australia's acting leader has cautioned that the search is a "challenge" and officials are "clutching" at clues.
Flight MH370 disappeared on 8 March while en route from Kuala Lumpur to Beijing, with 239 people on board.
Two Chinese military planes have flown out to the search area, around 2,500 km (1,550 miles) south-west of Australian city Perth, while two Japanese P-3 Orion aircraft will set off later on Monday.
They will join six other planes, including US and Australian military planes, to search a 68,500 sq km (26,000 sq miles) area in the ocean.
An Australian navy ship is already in the area, while several Chinese ships are also on their way.
However, the Australian Maritime Safety Authority, which is co-ordinating search efforts in the south, said on Monday: "The weather forecast in the search area is expected to deteriorate, with rain likely."
Warren Truss, Australia's acting prime minister while Tony Abbott is abroad, said that a tropical cyclone north of the search area could "stir up less favourable weather" for the search.
The search for flight MH370 has been in two large corridors - one stretching to the north-west of the last known location in the Malacca Straits and one to the south-west.
However, none of the countries on the northern corridor have reported any radar contact, and the Australian and Chinese satellite images of possible debris in the south Indian Ocean have concentrated the search there.
On Sunday, French officials said radar echoes from a satellite also identified possible debris in the south Indian Ocean.
The objects identified by the French satellite were "about 850 km north of our current search area", Truss told Australian broadcaster ABC.
"That's not in the area that had been identified as the most likely place where the aircraft may have entered the sea. But having said all that we've got to check out all the options."
Truss added that it was not certain that the plane was in the area. "We're just... clutching at whatever little piece of information comes along to try and find a place where we might be able to concentrate the efforts," he said.
On Monday, the US Pacific Command said it was moving a black box locator to the region "as a precautionary measure in case a debris field is located".
The equipment, which has a "highly sensitive listening capability", detects pings from the black box while being towed behind a ship.
The move did not indicate that debris had been found, the US added, but would enable search officers to "respond as quickly as possible" if debris is found, "since the battery life of the black box's pinger is limited".