An international search for missing Malaysia Airlines flight MH370 on Sunday began its fourth day covering an expansive area in the southern Indian Ocean.

The search zone was reaching about 2,500 km southwest of the Australian city of Perth, ABC News reported.

The search was focusing on finding an object measuring 22 meters by 13 meters spotted in Chinese satellite photos on March 18.

Experts believe its size and shape could be consistent with the wing of a Boeing 777 aircraft.

Australian Prime Minister Tony Abbott told ABC News on Sunday that two Chinese aircraft and two Japanese Orion aircraft would join the fleet flying into the search zone, which was expanded to 36,000 square km on Saturday.

The Australian Maritime Safety Authority (AMSA) said earlier Sunday that it had plotted the position of the object in the Chinese photos and it had fallen within Saturday's search area, although it was not sighted then.

The Chinese satellite photos were taken two days after American satellite photos that showed another object measuring 24 meters long in waters about 1,400 km southwest of the Australian city of Perth.

The locations of the photos were about 120 km apart.

The two Chinese air force Ilyushin IL-76 cargo planes and the Japanese aircraft have joined Australian and New Zealand air force P3 Orions, as well as two ultra-ling range civil aircraft in the search.

An AMSA statement said one of the civil aircraft reported Saturday sighting a number of small objects with the naked eye, including a wooden pallet, within a radius of 5 km.

A New Zealand air force P3 Orion aircraft with specialist electro-optic observation equipment was diverted to the location, arriving after the first aircraft left, but only reported sighting clumps of seaweed.

(Xinhua News Agency March 23, 2014)