CEO Ahmad Jauhari Yahya said at a news conference that Flight MH370 lost contact with Malaysian air traffic control at 2:40 a.m. (18:40 GMT Friday), about two hours after it had taken off from Kuala Lumpur. The plane, which carried passengers mostly from China but also from other Asian countries, North America and Europe, had been expected to land in Beijing at 6:30 a.m. Saturday (22:30 GMT Friday).
Pham Hien, a Vietnamese search and rescue official, said the last signal detected from the plane was 120 nautical miles (140 miles; 225 kilometers) southwest of Vietnam's southernmost Ca Mau province, which is close to where the South China Sea meets the Gulf of Thailand.
Lai Xuan Thanh, director of Vietnam's civil aviation authority, said air traffic officials in the country never made contact with the plane.
The plane "lost all contact and radar signal one minute before it entered Vietnam's air traffic control," Lt. Gen. Vo Van Tuan, deputy chief of staff of the Vietnamese army, said in a statement issued by the government.
The South China Sea is a tense region with competing territorial claims that have led to several low-level conflicts, particularly between China and the Philippines. That antipathy briefly faded as nations of the region rushed to aid in the search, with China dispatching two maritime rescue ships and the Philippines deploying three air force planes and three navy patrol ships to help.
"In times of emergencies like this, we have to show unity of efforts that transcends boundaries and issues," said Lt. Gen. Roy Deveraturda, commander of the Philippine military's Western Command.
The Malaysian Airlines plane was carrying 227 passengers, including two infants, and 12 crew members, the airline said. It said there were 153 passengers from China, 38 from Malaysia, seven from Australia, four from the U.S. and others from Indonesia, France, New Zealand, Canada, Ukraine, Russia, Italy, Taiwan, the Netherlands and Austria.
At Beijing's airport, authorities posted a notice asking relatives and friends of passengers to gather to a hotel about 15 kilometers (nine miles) from the airport to wait for further information, and provided a shuttle bus service. A woman wept aboard the shuttle bus while saying on a mobile phone, "They want us to go to the hotel. It cannot be good!"
In Kuala Lumpur, family members gathered at the airport but were kept away from reporters.
"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," Yahya, the airline CEO, said in a statement. "Our thoughts and prayers are with all affected passengers and crew and their family members."
Fuad Sharuji, Malaysian Airlines' vice president of operations control, told CNN that the plane was flying at an altitude of 35,000 feet (10,670 meters) and that the pilots had reported no problem with the aircraft.
Finding planes that disappear over the ocean can be very difficult. Airliner "black boxes" — the flight data and cockpit voice recorders — are equipped with "pingers" that emit ultrasonic signals that can be detected underwater.
Under good conditions, the signals can be detected from several hundred miles away, said John Goglia, a former member of the U.S. National Transportation Safety Board. If the boxes are trapped inside the wreckage, the sound may not travel as far, he said. If the boxes are at the bottom of a deep in an underwater trench, that also hinders how far the sound can travel. The signals also weaken over time.
Air France Flight 447, with 228 people on board, disappeared over the Atlantic Ocean en route from Rio de Janiero to Paris on June 1, 2009. Some wreckage and bodies were recovered over the next two weeks, but it took nearly two years for the main wreckage of the Airbus 330 and its black boxes to be located and recovered.
Malaysia Airlines said the 53-year-old pilot of Flight MH370, Zaharie Ahmad Shah, has more than 18,000 flying hours and has been flying for the airline since 1981. The first officer, 27-year-old Fariq Hamid, has about 2,800 hours of experience and has flown for the airline since 2007.
The tip of the wing of the same Malaysian Airlines Boeing 777-200 broke off Aug. 9, 2012, as it was taxiing at Pudong International Airport outside Shanghai. The wingtip collided with the tail of a China Eastern Airlines A340 plane. No one was injured.
Malaysia Airlines' last fatal incident was in 1995, when one its planes crashed near the Malaysian city of Tawau, killing 34 people. The deadliest crash in its history occurred in 1977, when a domestic Malaysian flight crashed after being hijacked, killing 100.
In August 2005, a Malaysian Airlines 777 flying from Perth, Australia, to Kuala Lumpur suddenly shot up 3,000 feet before the pilot disengaged the autopilot and landed safely. The plane's software had incorrectly measured speed and acceleration, and the software was quickly updated on planes around the world.
Malaysia Airlines has 15 Boeing 777-200s in its fleet of about 100 planes. The state-owned carrier last month reported its fourth straight quarterly loss and warned of tougher times.
The 777 had not had a fatal crash in its 19-year history until an Asiana Airlines plane crashed in San Francisco in July 2013. All 16 crew members survived, but three of the 291 passengers, all teenage girls from China, were killed.