Malaysia Airlines says it has lost contact with a plane travelling from Kuala Lumpur to Beijing, with 239 people on board.
The airline said in a statement that flight MH370 disappeared at 02:40 local time on Saturday (18:40 GMT on Friday).
It was expected to land in Beijing at 06:30 (22:30 GMT).
"Malaysia Airlines is currently working with the authorities who have activated their search and rescue team to locate the aircraft," the carrier said.
The Boeing B777-200 aircraft was carrying 227 passengers, including two children, and 12 crew members.
It went missing two hours after taking off from Kuala Lumpur.
It was lost in airspace controlled by Vietnam, China's Xinhua state news agency said.
The aircraft did not enter airspace controlled by China and did not make contact with Chinese controllers, the agency added.
Malaysia Airlines has told the BBC it will be holding a press conference later on Saturday.
The airline is the national carrier of Malaysia and one of Asia's largest, flying nearly 37,000 passengers daily to some 80 destinations worldwide.
Source: BBC news
Not looking good, by now the plane is probably out of fuel so it's looking more and more like a crash.