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.
Malaysian police named one man as Pouria Nour Mohammad Mehrdad, 18, and said he was probably migrating to Germany.
Interpol identified the other 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.
Interpol Secretary General Ronald Noble said the two men had travelled from Qatar’s capital Doha on their Iranian passports, and switched to stolen Italian and Austrian passports to board the Malaysia Airlines flight.