UPDATE: Australians are feared dead after a Malaysia Airlines flight carrying 239 people went missing between Kuala Lumpur and Beijing.
Two New Zealanders were also on flight MH370 which lost contact with Subang Air Traffic Control two hours after it departed Kuala Lumpur at 12.41am local time (12.41am Perth time).
Malaysia Airlines chief executive officer Ahmad Jauhari Yahya said the airline was notifying the next of kin, in a sign it expected the worst.
The 227 passengers were made up of 14 nationalities including 152 Chinese passengers and one infant, 38 Malaysians, 12 Indonesians, three passengers and an infant from the US, three French, two each from Ukraine and Canada and one each from Russia, Italy, Taiwan, The Netherlands and Austria.
There were conflicting reports about the number of Australians aboard the plane.
Mr Yahya said there were seven Australians, but Malaysia Airlines later revised the number down to six.
There were 12 crew members on board.
"We are deeply saddened with the news on MH370," Mr Yahya told reporters at a press conference in Malaysia this morning.
"Our thoughts and prayers are with all affected passengers and crews and their families."
The plane was piloted by 53-year-old Captain Zaharie Ahmad Shah from Malaysia.
He has about 18,300 hours of flying experience and has been flying for Malaysia Airlines since 1981.
First Officer Fariq Ab.Hamid, 37, also from Malaysia, joined the company in 2007.
Authorities activated a search and rescue team to locate the aircraft.
Reports that the plane had landed in the Nanming district of China proved to be false and reports that a signal from the plane indicated it had landed in the sea off the coast of Vietnam were being verified.
The Department of Foreign Affairs and Trade released a statement saying they were in "urgent and ongoing contact" with the airline.
The plane was a Boeing 777-200.
The airline's Kuala Lumpur-Beijing route passes over the Indochinese peninsula.
A report by China's Xinhua news agency said contact was lost with the plane while it was over Vietnamese airspace.
Xinhua also quoted Chinese aviation authorities saying the plane did not enter China's air traffic control sphere.
China's foreign ministry spokesman Qin Gang said in a statement: "We are very concerned learning this news."
"We are contacting relevant authorities and are trying to confirm relevant information."
A Beijing airport spokeswoman said the facility had activated an emergency response system.
Screens at the airport indicated the flight was "delayed".
Boeing, which has been beset by problems with its high-tech 787 Dreamliners put into service two years ago, including a months-long global grounding over battery problems last year, issued a brief statement on its Twitter feed.
"We're closely monitoring reports on Malaysia flight MH370. Our thoughts are with everyone on board," it said.
Consular officials are in contact with the families of the six Australians aboard the missing Malaysia Airlines jet.
“Australian consular officials are currently in touch with Malaysia Airlines and with the families of the missing Australian passengers,” Senator Brett Mason, parliamentary secretary to the Minister for Foreign Affairs, told reporters in Sydney.
“At the moment there is no clarity as to what has occurred.
“But can I just add that the families of the seven missing Australian passengers must be desperately concerned and the thoughts of the Australian Government and, I’m sure, all Australians go out to them at the moment.”
Senator Mason said he could not offer any further details about the age and gender of the Australians missing aboard flight MH370.
There had been no request for Australian authorities to join the search and rescue operation.