The plane lost communication two hours into the flight over Vietnam at 1:20 a.m. (18:20 GMT Friday), China's state news agency said. The radar signal also was lost, Xinhua reported.
There were rumours the plane had landed safely, but Fuad Sharuji, Malaysian Airlines' vice-president of operations control, told CNN that they were untrue and the airline had no idea where the plane was. AirAsia CEO Tony Fernandes sent a tweet saying that the radio failed and all were safe, but the tweet was later deleted.
Sharuji said that the plane was flying at an altitude of 35,000 feet and that the pilots reported no problem with the aircraft.
Flight MH370 departed Kuala Lumpur at 12:41 a.m. Saturday (16:41 GMT Friday) and was expected to land in Beijing at 6:30 a.m. Saturday (22:30 GMT Friday), Malaysia Airlines said.
The plane was carrying 227 passengers, including two infants, and 12 crew members, the airline said. It said two were Canadian, but provided no other details.
The airline said it was working with authorities who activated their search and rescue teams to locate the aircraft. The route would take the aircraft from Malaysia across to Vietnam and China.
"Our team is currently calling the next-of-kin of passengers and crew. Focus of the airline is to work with the emergency responders and authorities and mobilize its full support," Malaysia Airlines CEO Ahmad Jauhari Yahya said in a statement.
"Our thoughts and prayers are with all affected passengers and crew and their family members," he added.
At Beijing's airport, Zhai Le was waiting for her friends, a couple, who were on their way back to the Chinese capital on the flight. She said she was very concerned because she hadn't been able to reach them.
Airport authorities posted a written notice asking relatives and friends of passengers to gather to a hotel about 30 minutes drive from the airport to wait for further information, and provided a shuttle bus service.
Another woman wept aboard the shuttle bus while talking by mobile phone, " They want us to go to the hotel. It cannot be good!"
Malaysia Airlines has 15 Boeing 777-200 jets in its fleet of about 100 planes. The state-owned carrier last month reported its fourth straight quarterly loss.
The 777 had not had a fatal crash in its 20 year history until the Asiana crash in San Francisco in July 2013. All 16 crew members survived, but three of the 291 passengers, all teenage girls from China, were killed.___
Associated Press writers Chris Brummitt in Hanoi, Vietnam, Didi Tang and video producer Aritz Parra in Beijing, China, contributed to this report.
Correction: An earlier version of this article incorrectly stated the San Francisco Asiana crash occurred on July 2014. This version has been updated.
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