Malaysia Airlines Boeing 777 has reportedly crashed into the sea off Vietnam with 239 people on board.
It took off from Kuala Lumpur and was bound for Beijing.
Vietnamese state media has reported that the aircraft came down near Tho Chu Island. Malaysian authorities have not yet confirmed the crash and say no wreckage has been found. A huge search is underway.
Contact was lost two hours into the flight and anxious relatives at Beijing airport were initially told it had been delayed before confirmation came that it was missing.
The plane disappeared without giving a distress signal – a chilling echo of an Air France flight that crashed into the South Atlantic on June 1, 2009, killing all 228 people on board. It vanished for hours without issuing a distress call.
The vast majority of those on board Malaysia Airlines Flight 370 were Chinese but passengers were of 14 nationalities, among them Malaysians, Australians, Americans and French citizens.
If the crash is confirmed, it would mark the second fatal accident involving a Boeing 777 in less than a year.