The plane, a, left Kuala Lumpur 41 minutes after midnight on Saturday, and had been due to arrive in Beijing at 6.30 am local time (10.30 pm GMT Friday).
"We deeply regret that we have lost all contacts with flight MH370," the carrier said . "Focus of the airline is to work with the emergency responders and authorities and mobilize its full support. Our team is currently calling the next-of-kin of passengers and crew. Our thoughts and prayers are with all affected."
The flight was carrying 227 passengers from 13 different nationalities, including two infants, and 12 crew members, the airline said.
A report 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 airspace.
China 'very concerned'
China's foreign ministry spokesman, Qin Gang, said in a statement on Saturday: "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 Malaysia Airlines (MAS), which has bled money for years as its struggles to fend off competition from rivals such as fast-growing AirAsia.
It recorded its fourth straight quarterly loss during the final three months of 2013 and warned of a "challenging" year ahead due to intense competition.
In 2012, the carrier admitted it was in "crisis," forcing it to implement a cost-cutting campaign centred on slashing routes and other measures.
In 2011, it chalked up a record 2.5 billion ringgit ($767 million) loss.
Few crashes
The airline, which has 15 Boeing 777-200 jets in its fleet of about 100 planes, has had few crashes.
One of its jets crashed in 1977 in southern Malaysia, killing all 93 passengers and seven crew.
A smaller Twin Otter aircraft, operated by its unit MASwings, crashed upon landing in Malaysia's Sabah state on Borneo island last October, killing a co-pilot and a passenger.
The Boeing 777 had not had a fatal crash in its 20 year history until an in July 2013.
In December a Mozambique Airlines in a game park in northeast Namibia, killing all 34 people on board.
In November, a Russian passenger plane in the city of Kazan, killing all 50 people aboard.