EgyptAir flight MS804 from Paris to Cairo with 66 people on board has disappeared from radar, the airline says.
The Airbus A320 went missing over the eastern Mediterranean, soon after entering Egyptian airspace.
The Egyptian military has denied a report from EgyptAir that a distress signal was sent by the plane.
There were 56 passengers – including three children – seven crew members and three security personnel on board Flight MS804, the airline said.
The airline said the passengers included 30 Egyptians, 15 French citizens, one Briton, two Iraqis, as well as people from Canada, Belgium, Kuwait, Saudi Arabia, Algeria, Sudan, Chad and Portugal.
Both the Greek and Egyptian armed forces are involved in the search for the plane. France says it is sending boats and planes to help in the operation.
A Greek aviation official told the AFP news agency that the plane crashed “around 130 nautical miles” off the southern Greek island of Karpathos, although this has not been confirmed.