Military radar suggests the missing Malaysia Airlines plane turned west, away from its planned route, before vanishing, Malaysia’s air force says.
Flight MH370 from Kuala Lumpur to Beijing went missing on Saturday, after taking off with 239 people on board.
The international search for any wreckage has been widened.
Earlier, it emerged two men travelling on stolen passports on board the plane were Iranians with no apparent links to terrorist groups, officials said.
One of the men is believed to have been migrating to Germany.
The Malaysian authorities initially said flight MH370 disappeared about an hour after it took off from Kuala Lumpur International Airport (KLIA), as it flew over the South China Sea, south of Vietnam’s Ca Mau peninsula.
No distress signal or message was sent, but it is believed the plane attempted to turn back, perhaps towards Kuala Lumpur.
Officials still do not know what went wrong with the aircraft.