An EgyptAir flight carrying 66 passengers and crew on a flight from Paris to Cairo disappeared from radar over the Mediterranean Sea, Egypt's national airline said.
Egyptian Prime Minister Sherif Ismail said the search was underway to find the missing Airbus A320 and it was too early to rule out any explanation for the incident, including terrorism.
Officials with the airline and the Egyptian civil aviation department told Reuters they believed the jet had crashed into the sea.
However, it remained unclear whether the disappearance was due to technical failure or any other reason such as sabotage by ultra-hardline Islamists, who have targeted airports, airliners and tourist sites in Europe, Egypt, Tunisia and other Middle Eastern countries over the past few years.
Egypt Air said the plane sent an emergency signal - possibly from an emergency beacon attached to the plane - at 04:26 a.m., two hours after it disappeared from radar screens.
In water crashes, an underwater locator beacon attached to the aircraft's flight recorders starts to emit a signal or ping. This helps the search and rescue teams to locate the boxes, and the location of the crash.
The aircraft was carrying 56 passengers, including one child and two infants, and 10 crew, EgyptAir said. They included 30 Egyptian and 15 French nationals, along with nationals from 10 other countries.
"The theory that the plane crashed and fell is now confirmed after the preliminary search and after it did not arrive at any of the nearby airports," said a senior aviation source, who declined to be identified.
"All causes for the disaster are open, whether it is a major technical fault or a terrorist action or any other circumstance. This will be ascertained when we inspect the plane's wreckage and transcribe its black boxes."
Asked if he could rule out that terrorists were behind the incident, Prime Minister Ismail said: "We cannot exclude anything at this time or confirm anything. All the search operations must be concluded so we can know the cause."
"Search operations are ongoing at this time for the airplane in the area where it is believed to have lost contact," he told reporters at Cairo airport.
Egyptian President Abdel Fattah al-Sisi will chair a national security council meeting on Thursday morning, a statement from his office said. It did not say if the meeting would discuss the plane.
