'The aircraft did not suffer a significant loss of altitude,' she said.

Passengers and cabin crew were briefed on the likelihood of turbulence before the plane took off for the over five-hour night-flight, the spokeswoman said.

Severe turbulence has been cited as a possible cause of the Air France crash June 2 after the Airbus A330 ran into an equatorial storm on a flight from Rio de Janeiro to Paris although the reason for the tragedy remained unknown.

Clear-air turbulence is caused when bodies of air moving at greatly different speeds meet at high altitude. It is impossible to detect with the naked eye or with conventional aircraft equipment and has led to injuries and deaths in rare instances.