Australian Prime Minister Tony Abbott has warned that there is no clear end in sight in the search for missing Malaysia Airlines flight MH370, after the "pings" from the locator beacons on the plane's black box flight recorder fell silent.

It is believed that the batteries of the box, which records flight data and conversations from the cockpit, may have died.

The last four clear signals from the box were heard on 8 April, at a depth of 4,500 meters (15,000 feet) in the Indian Ocean.

"No one should underestimate the difficulties of the task still ahead of us," Abbott said in Beijing, on the last day of an official visit.

"There's still a lot more work to be done and I don't want anyone to think that we are certain of success, or that success, should it come, is going to happen in the next week or even month. There's a lot of difficulty and a lot of uncertainty left in this."

However, the signals already detected have allowed the search to be narrowed to a 500-square-mile patch of seabed.

If no more signals are heard and the area cannot be narrowed further, a robotic submersible will be sent to the seabed to scour the vast area.

It is believed that the Bluefin-21 submersible will take about six weeks to complete a search of the area.

After analysing satellite data, experts believe the plane went down in the southern Indian Ocean, off Australia's west coast, after veering off course for unknown reasons.

Up to 12 planes and 14 boats are searching 22,203 square miles of ocean about 1,367 miles north-west of Perth for signs of debris from the plane.