Some 25 countries are now involved in a vast search operation for the missing airliner that disappeared over a week ago, Malaysian officials say.

The search area - from central Asia to the southern Indian Ocean - takes in large tracts of land and sea.

An already complex search operation has become even more difficult, Malaysia's acting transport minister says.

Crew, passengers and ground staff are being investigated after it was confirmed the jet was commandeered.

Investigators are trying to obtain more radar and satellite data from any of the countries that Malaysia Airlines flight MH370 may have passed over, with its 239 crew and passengers.

The leaders of several Asian countries have been briefed by the Malaysia government in what acting Transport Minister Hishammuddin Hussein has described as a new phase of the search.

"From focusing mainly on shallow seas, we are now looking at large tracts of land, crossing 11 countries, as well as deep and remote oceans," he said at a news conference.

Malaysian officials are contacting countries including Kazakhstan, Uzbekistan, Kyrgyzstan, Turkmenistan, Pakistan, Bangladesh, India, China, Myanmar, Laos, Vietnam, Thailand, Indonesia, Australia and France.

They are also asking countries to provide assistance in the search for the plane, including satellite data and analysis, ground-search capabilities, and maritime and air assets.

After checking their radar recordings, Pakistani civil aviation officials said they had found no sign of the missing jet.

Malaysian national police chief Gen Khalid Abu Bakar said background checks had been requested on all passengers aboard the plane, but that so far nothing suspicious had been reported - though some intelligence agencies still had to respond.

The police are also reportedly looking at the family life and psychological state of the plane's pilot, Zaharie Shah, and co-pilot Fariq Abdul Hamid, and searched their homes on Saturday.

The two men had not asked to fly together, Mr Hishammuddin confirmed at the news conference.