No hypothesis can so far be ruled out about the fate of an EgyptAir plane that went missing earlier on Thursday on its way from Paris to Cairo, French President Francois Hollande said.
"Unfortunately the information we have ... confirms to us that the plane came down and is lost," Hollande said.
"No hypothesis can be ruled out, nor can any be favoured over another."
Separately, Paris prosecutors said that they were opening an investigation into matter.
Earlier, Egypt's Prime Minister Sherif Ismail said the search was under way to find the plane and it was too early to rule out any explanation for the incident, including terrorism.
"We cannot exclude anything at this time or confirm anything. All the search operations must be concluded so we can know the cause," he told reporters at Cairo airport.
Plane believed to have crashed off Greek island of Crete
Earlier, Egyptian and Greek officials said the plane is believed to have crashed in the Mediterranean Sea off the Greek island of Crete.
EgyptAir Flight 804 was lost from radar at around 2.45am local time (1045 AEST) when it was flying at 37,000 feet, according to the airline.
It said the Airbus A320 vanished 16 kilometres after it entered Egyptian airspace, around 280km off Egypt's coastline north of the Mediterranean port city of Alexandria.
Egyptian aviation officials said the plane crashed and that a search for debris was now under way.
The "possibility that the plane crashed has been confirmed" as the plane hasn't landed in any of the nearby airports, said the officials, who spoke on condition of anonymity.
Konstantinos Lintzerakos, director of Greece's Civil Aviation Authority, gave a roughly similar account to that given by EgyptAir.
In comments to the private Antenna television, he said Greek air traffic controllers were in contact with the pilot who reported no problems as the aircraft cruised at 37,000 feet, travelling at 830km/h.
The controllers tried to make contact with the pilot 16km before the plane exited the Greek Flight Information Region, or FIR.
The pilot did not respond, he said, and they continued to try to speak to him until 3.29am local time when the plane disappeared from the radar inside Egypt's FIR, 11km southeast of Crete.
Egypt's state-run newspaper Al-Ahram quoted an airport official as saying the pilot did not send a distress call, and that last contact with the plane was made 10 minutes before it disappeared from radar. It did not identify the official.
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No Australians believed to be on board
EgyptAir released information on the nationalities of the 56 passengers on board Flight MS804, including 30 Egyptians, 15 Frenchman, two Iraqis, and nine passengers each from Britain, Belgium, Kuwait, Saudi Arabia, Sudan, Chad, Portugal, Algeria and Canada.
DFAT said earlier the embassies in Cairo and Paris were trying to confirm whether there were any Australians on the plane.
Egyptian military aircraft and navy ships were taking part in a search operation off Egypt's Mediterranean coast to locate the debris of the plane, which was carrying 56 passengers, including one child and two babies, and 10 crew members.
Greece also joined the search and rescue operation, officials at the Hellenic National Defence General Staff said.
French Foreign Minister Jean-Marc Ayrault offered to send military planes and boats to join the Egyptian search for wreckage.
"We are at the disposition of the Egyptian authorities with our military capacities, with our planes, our boats to help in the search for this plane," he said.
He spoke after French President Francois Hollande held an emergency meeting at the Elysee Palace.
Later, the French military said a Falcon surveillance jet monitoring the Mediterranean for migrants had been diverted to help search for the EgyptAir plane.
Military spokesman Colonel Gilles Jaron told The Associated Press that the jet is joining the Egypt-led search effort, and the French navy may send another plane and a ship to the zone.
Hollande spoke with Egyptian President Abdel-Fattah el-Sissi on the phone and agreed to "closely co-operate to establish as soon as possible the circumstances" surrounding the incident, according to a statement issued in Paris.
In Cairo, el-Sissi convened an emergency meeting of the National Security Council, the country's highest security body. The council includes the prime minister and the defence, foreign and interior ministers, in addition to the chiefs of the intelligence agencies.
An Airbus A321 operated by Russia's Metrojet crashed in the Sinai in 31 October 2015, killing all 224 people on board. Russia and Western governments have said the plane was likely brought down by a bomb, and the Islamic State militant group said it had smuggled an explosive on board.
Reuters reported in January that an EgyptAir mechanic, whose cousin joined Islamic State in Syria, is suspected of planting the bomb, according to sources familiar with the matter.
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.
