Vanished after reaching 35,000 feet
Flight MH370, operating a Boeing 777-200ER aircraft, last had contact with air traffic controllers 120 nautical miles off the east coast of the Malaysian town of Kota Bharu, Malaysia Airlines chief executive Ahmad Jauhari Yahya said in a statement read to an earlier news conference in Kuala Lumpur.
Flight tracking website flightaware.com showed the plane flew northeast over Malaysia after takeoff and climbed to an altitude of 35,000 feet. The flight vanished from the website's tracking records a minute later while it was still climbing.
Malaysia and Vietnam were conducting a joint search and rescue, he said but gave no details. China and the Philippines have sent ships to the South China Sea to help in any search and rescue.
"We are extremely worried," Chinese Foreign Minister Wang Yi told reporters in Beijing before the initial Vietnamese report that the plane had crashed. "The news is very disturbing. We hope everyone on the plane is safe."
The flight left Kuala Lumpur at 12.21 a.m. (1621 GMT Friday) but no trace had been found of the plane more than eight hours after it was due to land in the Chinese capital at 6.30 a.m. (2230 GMT Friday) the same day.
"We deeply regret that we have lost all contacts with flight MH370," Jauhari said.
(Read more: Asian Airlines changing pilot training after US crash)
Malaysia Airlines said people from 14 nationalities were among the 227 passengers, including at least 152 Chinese, 38 Malaysians, 12 Indonesians, six Australians and three Americans. It also said a Chinese infant and an American infant were aboard.
If it is confirmed that the plane has crashed, the loss would mark the second fatal accident involving a Boeing 777 in less than a year and by far the worst since the jet entered service in 1995.
An Asiana Airlines Boeing 777-200ER crash-landed in San Francisco in July 2013, killing three passengers and injuring more than 180.
Boeing said it was aware of reports that the Malaysia Airlines plane was missing and was monitoring the situation but had no further comment. The flight was operating as a China Southern Airlines codeshare.
An official at the Civil Aviation Authority of Vietnam (CAAV) said the plane had failed to check in as scheduled at 1721 GMT while it was flying over the sea between Malaysia and Ho Chi Minh city.