Flight MH370, operating a Boeing B777-200 aircraft, departed Kuala Lumpur at 12.21 a.m. local time Saturday and was expected to land in Beijing at 6.30 a.m. the same day.
Malaysia Airlines said on its website that on board the plane were 12 crew members and 227 passengers, including five Indians, 154 Chinese and 38 Malaysians.
The Civil Aviation Administration of China has been informed by Vietnamese civil aviation authorities that Vietnam, Malaysia and Singapore are conducting a joint search south of Vietnam's Tho Chu Islands, Xinhua reported.
China has dispatched two maritime rescue ships to the South China Sea to help in rescue work.
"Earlier today, Subang ATC had lost contact with the aircraft at 2.40 a.m. The last known position of MH370 before it disappeared off the radar was 065515 North (longitude) and 1033443 East (latitude)," Malaysia Airlines said in a statement.
The airliner said the company was working with international authorities on the search and rescue mission.
In its latest website update issued at 7.20 p.m. Malaysia time, it said: "An international search and rescue mission was mobilised this morning. At this stage, our search and rescue teams from Malaysia, Singapore and Vietnam have failed to find evidence of any wreckage."
It added that the sea mission would continue while the air mission would recommence at daylight.
Malaysia Airlines group chief executive Ahmad Jauhari Yahya said the missing aircraft made its last contact about 200 km east of the Malaysian town of Kota Baru.
"We have contacted both the Malaysian and Vietnamese authorities as it is actually the borderline of Malaysian airspace and Vietnam," he told a press conference, adding there had been no indication the plane was in distress.
The flight had lost contact with ground control when flying into the Ho Chi Minh air traffic control area in Vietnam, authorities said.
Its signal never appeared to Ho Chi Minh City controllers.
Malaysian Transport Minister Hishamuddin Hussein said no sign of any plane wreckage had been found, denying earlier media reports that the plane had crashed south of an island off Vietnam. A Vietnamese newspaper had said that the aircraft had crashed into waters off Vietnam's southern Phu Quoc Island.
Hishamuddin said the Malaysian government had contacted the departments of maritime affairs of China, Vietnam, the Philippines and Indonesia to jointly search for the vanished plane.
Malaysian authorities dispatched a plane, two helicopters and four vessels to search seas off its east coast in the South China Sea, said Faridah Shuib, a spokesperson for the Malaysian Maritime Enforcement Agency.
Vietnam's defence ministry has also launched a search, a statement on the official Vietnamese government website said.
Singapore has sent a C130 Hercules aircraft for a search and locate mission for the missing plane, the city-state's air force said Saturday, while the Philippines' military said it had dispatched naval and air assets to join the international efforts.
The flight was piloted by Captain Zahaire Ahmad Shah, a Malaysian aged 53. He has a total of 18,365 flying hours to his credit and had joined Malaysia Airlines in 1981.
Fariq Ab. Hamid, 27, also a Malaysian, served as the first officer of the flight. He has logged 2,763 flying hours, the airline said.