According to military radars, 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.
None of the debris and oil slicks spotted in the South China Sea or Malacca Strait so far have proved to be linked to the disappearance.
Two-thirds of the passengers were Chinese. Others were from various Asian countries, North America or Europe.
Relatives have expressed frustration at the lack of information about the plane’s fate.
At least 40 ships and 34 aircraft are taking part in the search in the seas off Vietnam and Malaysia.
Search teams from Australia, China, Thailand, Indonesia, Singapore, Vietnam, Philippines, New Zealand and the United States of America are assisting.
The search is being conducted on both sides of the Malay Peninsula.
The area has been expanded from 50 nautical miles (57 miles) from where the plane disappeared – over waters between Malaysia and Vietnam – to 100 nautical miles.
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