Malaysia Airlines said in a statement that flight MH370 had disappeared at 02:40 local time on Saturday (18:40 GMT on Friday) after leaving Kuala Lumpur.
It had been expected to land in Beijing at 06:30 (22:30 GMT).
Malaysia's transport minister said there was no information on wreckage and he urged against speculation.
"We are doing everything in our power to locate the plane. We are doing everything we can to ensure every possible angle has been addressed," Seri Hishammuddin told reporters in Kuala Lumpur.
"Our hope is that the people understand we are being as transparent as we can, we are giving information as quickly as we can, but we want to make sure information has been verified."
Malaysia Airlines chief executive Ahmad Jauhari Yahya said the focus was on helping the families of those missing. He said that 80% of the families had been contacted.
The plane went off the radar south of Vietnam, according to a statement on the Vietnamese government website.
Its last known location was off the country's Ca Mau peninsular although the exact position was not clear, it said.
The Boeing B777-200 aircraft was carrying 227 passengers, including two children, and 12 crew members.
A plane, two helicopters and four vessels have been dispatched by Malaysia to search the seas off its east coast in the South China Sea, the Malaysian Maritime Enforcement Agency was quoted as saying by AFP news agency.
Vietnam also launched a search while the Philippines said it was sending three navy patrol boats and a surveillance plane, AFP adds, and China sent two ships.
The passengers were of 14 different nationalities, Mr Jauhari said.
Among them were 152 Chinese nationals, 38 Malaysians, 12 people from Indonesia and six from Australia.
The pilot was Capt Zaharie Ahmad Shah, 53, who joined Malaysia Airlines in 1981, Mr Yahya said.
A Vietnamese navy official told the BBC the plane had gone missing within Malaysian maritime territory.
Friends and relatives expecting to meet passengers from the flight in Beijing were instructed to go to a nearby hotel where officials were meant to be on hand to provide support.
The Associated Press reported a woman weeping on a shuttle bus who was heard to say on a mobile phone: "They want us to go to the hotel. It cannot be good."
The plane had been flying at an altitude of 35,000ft (10,700m) and the pilots had not reported any problems with the aircraft, Fuad Sharuji, Malaysian Airlines' vice-president of operations control, told CNN.
Malaysia's national carrier is one of Asia's largest, flying nearly 37,000 passengers daily to some 80 destinations worldwide.
The route between Kuala Lumpur to Beijing has become more and more popular as Malaysia and China increase trade, says the BBC's Jennifer Pak in Kuala Lumpur.
The Boeing 777 had not had a fatal crash in its 20-year history until an Asiana plane came down at San Francisco airport in July of last year. Three teenage girls from China died in that incident.
Boeing said in a statement posted on Twitter: "We're closely monitoring reports on Malaysia flight MH370. Our thoughts are with everyone on board."
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