Rescue crews have put no time limit on the search for the missing Malaysia Airlines plane, Australian Prime Minister Tony Abbott has said.Mr Abbott told reporters near Perth, where the operation is being co-ordinated, that the hunt for flight MH370 was still being stepped up.

Ten aircraft and 10 ships are scouring the sea south-west of Perth for debris from the airliner.

The Beijing-bound plane disappeared on 8 March with 239 people on board.

The signal from its flight-data recorder lasts about 30 days.

The search teams are deploying a special tool known as a “towed ping locator” to find the recorder, which will be used once debris from the plane has been found.

Several floating objects have been found during the search in recent days, but none is believed to belong to the missing plane.

“We can keep searching for quite some time to come,” said Mr Abbott.

“The intensity of our search and the magnitude of our search is increasing, not decreasing.”

Some 153 of the passengers were Chinese, and dozens of their relatives arrived in Kuala Lumpur on Sunday.

They have become increasingly upset with the perceived lack of information from the Malaysian authorities.

Chanting “Tell us the truth”, they said they wanted Malaysian Prime Minister Najib Razak to apologise for what they regard as misleading statements.

Many were outraged when Mr Najib stated earlier that he believed the plane had come down in the Indian Ocean with no survivors.

But Mr Abbott gave his backing to that assessment, saying: “The accumulation of evidence is that the aircraft has been lost and it has been lost somewhere in the south of the Indian Ocean.

“That’s the absolutely overwhelming wave of evidence and I think that Prime Minister Najib Razak was perfectly entitled to come to that conclusion.”

Various theories about what went wrong have been suggested – including the captain hijacking his own plane.

The speculation was fuelled by reports that files had been deleted on the pilot’s home flight simulator.

However, on Saturday Malaysia’s transport minister said investigators had found “nothing sinister” from the simulator.

Malaysia Airlines Boeing 777 vanished less than an hour after taking off from Kuala Lumpur.

The airliner diverted off course and lost contact with air traffic controllers between Malaysian and Vietnamese air-traffic control areas.