Search planes flying deep into the southern Indian Ocean have found nothing so far that could be from the missing Malaysia Airlines jet, Australia's acting prime minister said.

The planes are part of an international effort to solve the nearly two-week-old mystery of what happened to Flight mh370 and the 239 people on board.

They are looking for two large floating objects - one 80ft long, the other 15ft - in an area 1400 miles off the south-west coast of Australia, about halfway to the desolate islands of the Antarctic. The objects were detected by satellite.

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"It's about the most inaccessible spot you could imagine on the face of the earth, but if there is anything down there we will find it. We owe it to the families of those people to do no less," said Warren Truss, who is acting prime minister while Tony Abbott is in Papua New Guinea.

"The last report I have is that nothing of particular significance has been identified in the search today but the work will continue,"

Mr Truss said the search was difficult due to testing weather conditions and because the satellite imagery was five days old.

He said: "So something that was floating on the sea that long ago may no longer be floating - it may have slipped to the bottom. It's also certain that any debris or other material would have moved a significant distance over that time, potentially hundreds of kilometres.

"We can't be certain the sightings are debris from the aircraft, but it is about the only lead around at the present time."

Two Chinese aircraft are expected to arrive in Perth today to join the search, and two Japanese aircraft will be arriving tomorrow. A small flotilla of ships coming to Australia from China is still several days away.

The search area is so remote it takes aircraft four hours to fly there and four hours back, leaving them only enough fuel to search for about two hours. The last plane to return to Australia late last night was a US P8 Poseidon aircraft.

Malaysian defence minister Hishammuddin Hussein said he would be speaking to us Secretary of Defence Chuck Hagel later "to request further specialist assets to help with the search and rescue efforts, including remotely operated vehicles for deep ocean salvage".

The search area will change slightly today depending on water movements overnight, said John Young, manager of the Australian Maritime Safety Authority's emergency response division.

Mr Abbott spoke with Chinese president Xi Jinping, whom he described as "devastated". Of the 227 passengers on the missing flight, 154 were from China.

The Norwegian cargo vessel Hoegh St Petersburg is also in the area helping with the search. The ship, which transports cars, was on its way from South Africa to Australia.

The Maritime Safety Authority said another commercial ship and an Australian navy vessel were en route.

Three Chinese naval ships were heading to the area, along with the icebreaker Snow Dragon, Chinese state TV reported. The icebreaker was already in Perth following a voyage to Antarctica in January.

The planes are using radar to detect objects and then making low passes over the ocean to identify them visually, because when "radar blips come back it's not always clear what the object is," said Michael Smart, an aerospace engineering professor at Australia's University of Queensland.

Malaysian authorities have said the evidence so far suggests the plane was deliberately turned back across Malaysia to the Strait Of Malacca, with its communications systems disabled. They are unsure what happened next.

Police are considering the possibility of hijacking, sabotage, terrorism or issues related to the mental health of the pilots or anyone else on board.