China said on Saturday it had a new satellite image of what could be wreckage from a missing Malaysian airliner, as more planes and ships headed to join an international search operation scouring some of the remotest seas on Earth.

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

The new potential sighting was dramatically announced by Malaysia's acting transport minister, Hishammuddin Hussein, after he was handed a note with details during a news conference in Kuala Lumpur, scooping the official announcement from China.

"Chinese ships have been dispatched to the area," Hishammuddin told reporters.

China said the object was 22 metres long (74ft) and 13 metres (43ft) wide, and spotted around 120 km (75 miles) "south by west" of potential debris reported by Australia off its west coast in the forbidding waters of the southern Indian Ocean.

The image was captured by the high-definition Earth observation satellite "Gaofen-1" early on March 18, two days after the Australian satellite picture was taken, China's State Administration of Science, Technology and Industry for National Defense (SASTIND) said on its website.

There was no official comment on whether the two images could show the same object.

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

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

That has led them to focus on hijacking or sabotage, but they have not ruled out technical problems.

REMOTE SEAS

Since Australia announced the first image of what could be parts of the aircraft on Thursday, the international search for the plane has focused on an expanse of ocean more than 2,000 km (1,200 miles) southwest of Perth.

The Australian Maritime Safety Authority (AMSA) said one of its aircraft reported sighting a number of "small objects" with the naked eye, including a wooden pallet, within a radius of five km.

A Royal New Zealand Air Force P-3 Orion aircraft took a closer look but only reported seeing clumps of seaweed. It dropped a marker buoy to track the movement.

"A merchant ship in the area has been tasked to relocate and seek to identify the material," AMSA said in a statement.

The search area experienced good weather conditions on Saturday with visibility of around 10 kilometres and moderate seas.

Australia, which is coordinating the rescue, has cautioned the objects in the satellite image might be a lost shipping container or other debris, and may have sunk since the picture was taken.

"Even though this is not a definite lead, it is probably more solid than any other lead around the world and that is why so much effort and interest is being put into this search," Deputy Prime Minister Warren Truss told reporters, before latest Chinese image was reported.

China said its icebreaker "Snow Dragon" was heading for 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 southern search zone.

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," said Hishammuddin. "The ocean varies between 1,150 metres and 7,000 metres in depth."

NO SIGN IN NORTHERN CORRIDOR

Where the missing plane went after it flew out of range of Malaysia's military radar off the country's northwest coast has been one of the most puzzling aspects of what has quickly become perhaps the biggest mystery in modern aviation history.

Electronic "pings" detected by a commercial satellite suggested it flew for another six hours or so, but could do no better than place its final signal on one of two vast arcs: a northern corridor from Laos to the Caspian Sea, and a southern one stretching from Indonesia down to the part of the Indian Ocean that has become the focal point of the search.

Malaysia has said the search will continue in both corridors until confirmed debris is found.

Hishammuddin said that, in response to a formal diplomatic request from Malaysia, China, India, Pakistan, Myanmar, Laos, Kyrgyzstan and Kazakhstan had all said, based on preliminary analysis, that there have been no sightings of the aircraft on their radar.

Aircraft and ships have renewed the search in the Andaman Sea between India and Thailand, going over areas in the northern corridor that have already been exhaustively swept.

The Pentagon said it was considering a request from Malaysia for sonar equipment. The P-8 and P-3 spy planes, which the United States is already deploying in the search, also carry "sonobuoys" that are dropped into the sea and use sonar signals to search the waters below.

The search itself has strained ties between China and Malaysia, with Beijing repeatedly leaning on the Southeast Asian nation to step up its hunt and do a better job at looking after the relatives of the Chinese passengers.

For families of the passengers, the process has proved to be an emotionally wrenching battle to elicit information.

In a statement on Saturday, relatives in Beijing lambasted a Malaysian delegation for "concealing the truth" and "making fools" out of the families after they said they left a meeting without answering all their questions.

"This kind of conduct neglects the lives of all the passengers, shows contempt for all their families, and even more, tramples on the dignity of Chinese people and the Chinese government," they said.

Some experts have argued that the reluctance to share sensitive radar data and capabilities in a region fraught with suspicion amid China's military rise and territorial disputes may have hampered the search.