KUALA LUMPUR (AFP) - Malaysia Airlines (MAS) said the airline was notifying next of kin of 239 passengers onboard a flight that lost control with air control tower, in a sign that it expected the worst
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."
An accident would be a huge blow for MAS, which has bled money for years as its struggles to fend off competition from rivals such as fast-growing AirAsia.
The airline said flight MH370 was carrying 239 people from Kuala Lumpur to Beijing when it disappeared at 2.40 am local time, about two hours after leaving Kuala Lumpur International Airport. It had been due to arrive in Beijing at 6.30 am local time.
It was carrying 227 passengers, including two infants, from 13 different nationalities, and 12 crew members.
China's state television said 158 of the passengers were Chinese.
"We deeply regret that we have lost all contacts with flight MH370 which departed Kuala Lumpur at 12.41am earlier this morning bound for Beijing," Malaysia Airlines Group Chief Executive Officer Ahmad Jauhari Yahya said in a statement.
The statement said the Malaysian flag carrier was working with authorities, who had launched an effort to locate the aircraft.
"Our team is currently calling the next-of-kin of passengers and crew," Mr Ahmad Jauhari said.
"Our thoughts and prayers are with all affected passengers and crew and their family members."
The plane was a Boeing 777-200. The airline's Kuala Lumpur-Beijing route passes roughly over the Indochinese peninsula.
A Malaysian Airlines spokeswoman said she could not immediately provide further details.