* Flight MS804 missing soon after entering Egyptian airspace

* Airbus A320 carrying 56 passengers, 10 crew

* Officials believe it crashed into the sea

* Too early to rule out any cause, including terrorism- Egypt premier

* Aboard flight were 30 Egyptians, 15 French, 10 other nationalities

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, including terrorism.

Officials with the airline and the Egyptian civil aviation department told Reuters they believed the jet had crashed into the Mediterranean between Greece and Egypt.

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 beacon attached to the aircraft's flight recorders starts to emit a signal or ping. This helps search and rescue teams to locate the crash and find the boxes.

The aircraft was carrying 56 passengers - with one child and two infants among them - and 10 crew, EgyptAir said. They included 30 Egyptian and 15 French nationals, along with citizens of 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.



Families of passengers react as they wait outside a services hall at Cairo airport

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.

The pilot had clocked up 6,275 hours of flying experience, including 2,101 hours on the A320, while the first officer had 2,766 hours, the airline said.

No response

Greek air traffic controllers spoke to the pilot as the jet flew over the island of Kea, in what was thought to be the last broadcast from the aircraft, and no problems were reported.

But just ahead of the handover to Cairo airspace, calls to the plane went unanswered, before it dropped off radars shortly after exiting Greek airspace, Kostas Litzerakis, the head of Greece's civil aviation department, told Reuters.

"During the transfer procedure to Cairo airspace, about seven miles before the aircraft entered the Cairo airspace, Greek controllers tried to contact the pilot but he was not responding," he said.

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.

At Cairo airport, authorities ushered families of the passengers and crew into a closed-off waiting area.

However, two women and a man, who said they were related to a crew member, were seen leaving the VIP hall where families were being kept. Asked for details, the man said: "We don't know anything, they don't know anything. No one knows anything."

Ayman Nassar, from the family of one of the passengers, also walked out of the passenger hall with his daughter and wife in a distressed state. "They told us the plane had disappeared, and that they're still searching for it and not to believe any rumours," he said.

A mother of flight attendant rushed out of the hall in tears. She said the last time her daughter called her was Wednesday night. "They haven't told us anything," she said.

EgyptAir said on its Twitter account that Flight MS804 had departed Paris at 23:09 (CEST). It disappeared at 02:30 a.m. at an altitude of 37,000 feet (11,280 metres) in Egyptian air space, about 280 km (165 miles) from the Egyptian coast before it was due to land at 03:15 a.m..

"There was nothing unusual," EgyptAir vice chairman Ahmed Adel told Reuters. "The search and rescue aircraft from the Egyptian air force are at the position where we lost contact. They are still looking and so far there is nothing found."

In Paris, a police source said investigators were now interviewing officers who were on duty at Roissy airport on Wednesday evening to find out whether they heard or saw anything suspicious. "We are in the early stage here," the source said.

Airbus said the missing A320 had been delivered to   EgyptAir in November 2003 and had operated about 48,000 flight hours.

Greece said it had deployed aircraft and a frigate to the area to help with the search. A Greek defence ministry source said authorities were also investigating an account from the captain of a merchant ship who reported a 'flame in the sky' about 130 nautical miles south of the island of Karpathos.

France, Egypt to cooperate

The weather was clear at the time the plane disappeared, according to Eurocontrol, the European air traffic network. "Our daily weather assessment does not indicate any issues in that area at that time," it said.

Speed and altitude data from aviation website FlightRadar24.com indicated the plane was cruising at the time it disappeared.

French President Francois Hollande's office said the French leader had just spoken to his Egyptian counterpart and that both sides would cooperate closely.

Under UN aviation rules, Egypt will automatically lead an investigation into the accident assisted by countries including France, if it is confirmed that an Airbus jet was involved.

"We are in close contact with the Egyptian authorities, both civil and military," French Prime Minister Manuel Valls told French radio. "At this stage, no theory can be ruled out regarding the causes of the disappearance."

With its ancient archeological sites and Red Sea resorts, Egypt is a popular destination for Western tourists. But the industry was badly hit following the downing of a Russian jet last year, an Islamist insurgency and a string of bomb attacks in the country.

An Airbus A321 operated by Russia's Metrojet crashed in the Sinai on October 31, 2015, killing all 224 people on board. Russia and Western governments have said the plane was probably brought down by a bomb, and the Islamic State militant group said it had smuggled an explosive device on board.

The crash called into question Egypt's campaign to eradicate Islamist militancy and has damaged its tourism industry, a cornerstone of the economy.

Islamist militants have stepped up attacks on Egyptian soldiers and police since Sisi, as army chief, toppled freely elected Islamist President Mohamed Mursi in 2013 after mass protests against his rule.

In March, an EgyptAir plane flying from Alexandria to Cairo was hijacked and forced to land in Cyprus by a man with what authorities said was a fake suicide belt. He was arrested after giving himself up.

In the same month, Islamic State suicide bombers hit Brussels airport and a metro train in the worst such attacks in Belgian history, killing 32 people. Investigators believed they were carried out by the same cell that was behind November's gun and bomb attacks in Paris which claimed the lives of 130 people.

EgyptAir has a fleet of 57 Airbus and Boeing jets, including 15 of the Airbus A320 family of aircraft, according to airfleets.com.

The last fatal incident involving an EgyptAir aircraft was in May 2002, when a Boeing 737 crashed into a hill while on approach to Tunis-Carthage International Airport, killing 14 people.