China said yesterday it had a new satellite image of what could be wreckage from a missing airliner, as more planes and ships joined an international search operation scouring some of the world's remotest seas.

The latest lead came as the search for Malaysian Airlines flight MH370 entered its third week, with still no confirmed trace of the Boeing 777 or the 239 people on board.

The potential sighting was dramatically announced by Malaysia's acting transport minister, Hishammuddin Hussein, after he was handed a note during a news conference in Kuala Lumpur, scooping the announcement from China. "Chinese ships have been dispatched to the area," Hishammuddin said.

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China said the object was 74ft by 43ft, and spotted around 75 miles "south by west" of possible debris reported by Australia off its west coast in the southern Indian Ocean.

The image was captured by the high-definition observation satellite "Gaofen-1" early on March 18, two days after the Australian satellite picture was taken. It could not be determined from the blurred images whether the objects were the same.

The wing of a Boeing 777-200ER has approximately the same dimensions as the object observed in the Chinese images.

Flight MH370 vanished from civilian radar screens on March 8, less than an hour after taking off from Kuala Lumpur on a flight to Beijing.

Where the plane headed after it flew out of range of radar off Malaysia's northwest coast has been one of the most puzzling aspects of what is possibly the biggest mystery in modern aviation history.

Investigators believe someone on board shut off the plane's communications systems, and partial military radar tracking showed it turning west and recrossing the Malay Peninsula, apparently under the control of a skilled pilot.

As a result, hijacking or sabotage has been suspected, but technical problems have not been ruled out.

Since last Thursday, when Australia announced the first image of what could be parts of the aircraft, the search has focused on an expanse of ocean more than 1200 miles south-west of Perth.

The Australian Maritime Safety Authority (AMSA) said one of its aircraft reported sighting "small objects" with the naked eye, including a wooden pallet, within a 5km radius. A Royal New Zealand Air Force P-3 Orion aircraft took a closer look but reported seeing only clumps of seaweed. It dropped a marker buoy to track the movement.

Australia, which is co-ordinating the rescue, has cautioned the objects in the satellite image might be a lost shipping container or other debris. "A merchant ship in the area has been tasked to relocate and seek to identify the material," AMSA said in a statement.

China said its icebreaker Snow Dragon was heading to the area, but was still around 70 hours away.

Japan and India were also sending more planes and Australian, and Chinese navy vessels were steaming towards the search zone.

Good weather conditions in the search area yesterday meant visibility of around 10km and moderate seas.

But the area is known for rough seas and strong currents, and Malaysia's Hishammuddin said a cyclone warning had been declared for Christmas Island, far off to the north. "There are vessels heading in that direction. They may have to go through the cyclone," he said.

"Generally, conditions in the southern corridor are very challenging," he added. "The ocean varies between 1150 metres and 7000m in depth."