An EgyptAir flight from Paris to Cairo is thought to have crashed into the sea with 56 passengers and 10 crew on board.
Flight MS804 departed the French capital at 11.09pm (CEST) before vanishing, according to what the airline described as “an informed source”.
The airline said that the plane, which was carrying two babies and one child among its passengers, lost contact with radar at 2.45am Cairo time (1.45am BST). Its final contact with air control was 10 minutes earlier.
At that stage the Airbus A320, which was 13 years old, was about three hours and 40 minutes into the four-hour journey and flying at 37,000ft.
The airline said the plane had been 10 miles into Egyptian airspace, over the Mediterranean Sea, when it disappeared.
However, Egyptian civil aviation authority spokesman Ihab Raslan told Sky News Arabia that it was about to enter Egyptian airspace when it disappeared. He said the plane had most likely crashed into the sea.
Greek authorities have joined special teams from the Egyptian armed forces in the search for the jet.
Ahmed Abdel, the vice-chairman of EgyptAir holding company, told CNN that there had been no distress calls from the plane.
The New York Times quoted Ehab Mohy el-Deen, the head of Egypt’s air navigation authority, as saying that Greek air traffic controllers had told their Egyptian counterparts that they had lost contact with the plane.
He said: “They did not radio for help or lose altitude. They just vanished.”
Egypt’s Civil Aviation Ministry says that search and rescue teams, including those from the Egyptian Armed Forces, are looking for the plane.
Greece has also joined the search and rescue operation, sending one C-130 and one early warning aircraft, officials at the Hellenic National Defence General Staff said. They said one frigate was also heading to the area, and helicopters are on standby.
Meanwhile, an aviation source said the Civil Aviation Ministry was working to gather information on the technical state of the missing plane.
The airline said the plane’s pilot had flown 6,275 hours – including 2,101 hours on the same model – while the co-pilot had done 2,766 hours.
Aviation expert Rusty Aimer told Sky News: “Right now, the EgyptAir folks are talking with the Egyptian air traffic control facility.
“They’re in direct contact, trying to get as much information as they can, trying to find out – were there any radar outages in the sector?
“A radar outage… could be a loss of power to the radar station: it could be sabotage, it could be many things.
“That could be one reason they lost control of the aircraft. The other reason could be the aircraft itself disappearing, which could mean not a good ending.”
He added: “There are two scenarios where the radar blip of an aircraft disappears. The first is where the ground radar is not working for some reason and there’s a possibility of that happening.
“The second is if the aircraft itself loses the information it gives to the radar or the transponder.”
Sky News Middle East Correspondent Sherine Tadros said: “This is a popular route, this is a time when people would have wanted to make that trip for business reasons, for personal reasons.”
With its ancient archaeological sites and Red Sea resorts, Egypt is a popular destination for tourists.
But its tourism industry was badly hit following the downing of a Russian jet last year with the deaths of all 224 on board, the ongoing Islamist insurgency and a string of bomb attacks in the country.