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.
The international police organisation Interpol’s Tehran bureau has said the two Iranians had no criminal records and had left Iran legally. One of the men is believed to have been migrating to Germany.
There are reports that the plane veered from its route and reached the Malacca Strait, west of Malaysia.
The Malaysian authorities initially said flight MH370 disappeared about an hour after it took off from Kuala Lumpur International Airport, 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.
In the US, CIA Director John Brennan said the possibility of a terror link could not be ruled out. But he said “no claims of responsibility” over the missing jet had “been confirmed or corroborated”.
“Clearly this is still a mystery, which is very disturbing,” he said at the Council on Foreign Relations think-tank.
Two-thirds of the passengers on board the plane were Chinese. Some were from a range of other 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 US are assisting in the operation on both sides of the Malay Peninsula.
The area has been expanded from 50 nautical miles (57 miles; 93km) from where the plane disappeared – over waters between Malaysia and Vietnam – to 100 nautical miles.
Earlier, Malaysian police named one of the two men who travelled on the plane on a stolen passport as Pouria Nour Mohammad Mehrdad, 18, and said he was probably migrating to Germany.
Interpol identified the other man as Delavar Seyed Mohammadreza, 29.
Experts have said the presence of two people with stolen passports on a plane was a breach of security, but one that is relatively common in a region regarded as a hub for illegal migration.
Malaysian police say the younger Iranian was “not likely to be a member of a terrorist group”, adding that the authorities were in contact with his mother in Germany, who had been expecting her son to arrive in Frankfurt.
And Interpol says the two men travelled from Qatar’s capital Doha on their Iranian passports, and switched to stolen Italian and Austrian passports to board the Malaysia Airlines flight.