Bun Ratha, director of Siem Reap international airport, told the Reuters news agency: "We do not know the fate of the lost aircraft, but the authorities have sent helicopter search teams."
Him Sarun said the crash site had not yet been located by rescue teams.
Some reports said conservationists working on Kamchay Mountain had seen the crash.
An airport official said contact with the flight was lost at 10:50am (03:50GMT), five minutes before it was due to land.