The Egyptian president has called for an "intensified search" involving the army, navy and air force to find the debris of an EgpytAir plane that disappeared early today.
The move by Abdel Fattah al-Sisi comes after Greek authorities found "floating material" that is likely to be debris from the plane that crashed over the Mediterranean, as well as life jackets.
However, the airline's vice president has dismissed these claims, saying the wreckage has not been found.
Earlier the Egyptian aviation minister said EgyptAir flight MS804 was more likely brought down by a terror attack than technical failure.
Mr Sisi has ordered an investigative committee to immediately start investigating the cause of the plane's disappearance.
The plane was carrying 66 passengers and crew on a flight from Paris to Cairo.
Greek state television has reported that aircraft debris was found in the southern Mediterranean by Greek authorities.
Earlier, a Greek frigate searching for the aircraft discovered two large plastic floating objects in the sea 230 miles south of the island of Crete, Greek defence sources said this afternoon.
A US review of satellite imagery has not found any sign of explosion aboard the flight, according to officials from a US agency.
The two objects appeared to be pieces of plastic in white and red. It has also been reported that two life vests, which appeared to be from an aircraft, were also found.
Speaking during a press conference, minister Sherif Fathy said there is no known security issues with passengers but that further checks were under way.
He said: "The possibility of having a different action on board, of having a terror attack, it is higher than the possibility of having a technical failure." He added that he still had no information on the plane's fate.
Search efforts are focused near the Greek island of Karpathos as well as the vicinity just south of it, but he described the search as "flexible".
The plane was in Egypt airspace at 37,000 feet and made sudden swerves before disappearing from radar, the Greek defence minister said earlier.
French President Francois Hollande said that the Airbus A320 had "crashed".
"We must ensure that we know everything on the causes of what happened. No hypothesis is ruled out or favoured," he said in a televised address.
"Whether it was an accident or another hypothesis that everyone has on their mind - a terrorist hypothesis ... at this stage we must focus on our solidarity with the families and the search for the causes of the catastrophe," Mr Hollande added.
"We feel solidarity and compassion. It's not the first such catastrophe, and we know what it means for families and loved ones," Mr Hollande added.
Meanwhile, a meeting of Europe's leftist leaders, including President Hollande, has been cancelled following the disappearance of the EgyptAir aircraft. The meeting was due take place in Rome and Italian officials have confirmed that it has been postponed until a later date.
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The aircraft was carrying 56 passengers, including one child and two infants, and ten crew, EgyptAir said.
They included 30 Egyptian and 15 French nationals, along with nationals from ten other countries.
EgyptAir said the plane sent an emergency signal – possibly from an emergency beacon attached to the plane - at 4.26am local time (2.26am Irish time), 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.
Families of passengers and the Egyptian prime minister arrived at Cairo International Airport shortly after dawn while the Egyptian and Greek military scrambled aircraft and boats to search for the plane.
The airline said on its Twitter account that Flight MS804 departed Paris at 11.09pm CEST (10.09pm Irish time).
It disappeared at 2.30am local time at an altitude of 37,000ft in Egyptian airspace, some 280km from the Egyptian coastline, before it was due to land at 3.15am, the airline said.
"There was nothing unusual," EgyptAir vice chairman Ahmed Adel said.
"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."
Egyptian state newspaper Ahram reported no distress call was made and the last contact was ten minutes before the plane disappeared.
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 problems mentioned in last contact from plane
Greek air traffic controllers spoke to the pilot over the island of Kea, in what was thought to be the last broadcast from the aircraft.
"The pilot did not mention any problems," Kostas Litzerakis, the head of Greece's civil aviation department, said.
However, during the transfer to Cairo airspace, Greek controllers were unable to reach the pilot and the plane vanished from their radars shortly after leaving Greek airspace, he added.
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” around 130 nautical miles south of the island of Karpathos.
The weather was clear at the time the plane disappeared, according to weather reports.
Speed and altitude data from aviation website FlightRadar24.com indicated the plane was cruising at the time it disappeared.
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."
An informed source at EGYPTAIR stated that Flight no MS804,which departed Paris at 23:09 (CEST),heading to Cairo has disappeared from radar.
— EGYPTAIR (@EGYPTAIR) May 19, 2016
Last received ADS-B position from #MS804 with Egyptian FIR (Egyptian airspace) boundary overlay. pic.twitter.com/TCGyEM6zT7
— Flightradar24 (@flightradar24) May 19, 2016
Airbus' verified Twitter account said: "We are aware of media reports. At this time we have no further details, but we will provide further information when available."
An emergency response room has been set up at the Integrated Operations Control Centre in EgyptAir at Cairo airport.
EGYPTAIR has hosted the passengers' families near to Cairo Airport and has provided doctors, translators and all the necessary services.
— EGYPTAIR (@EGYPTAIR) May 19, 2016
Commercial airline pilot Chris McGee told Sky News there were only two circumstances that would prevent a pilot from contacting air traffic control.
"One would be if there was human intervention, if you are prevented from doing so by someone preventing you from doing that, which is a very, very, very unusual situation indeed," she said.
"The second possibility, which is also very unprecedented, is that something has occurred on the flight deck which means we are simply too busy, we have got to handle what is happening to the aircraft at that point in time and we do that first.
"The first thing you are taught is fly the airplane first, handle the problem, and then communicate. So if you have got your hands full that is potentially why you wouldn't talk to air traffic."
Egypt is a popular destination for Western tourists.
But the industry was badly hit following the downing of a Russian jet last year, the ongoing Islamist insurgency and a string of bomb attacks in the country.
An Airbus A321 operated by Russia's Metrojet crashed in the Sinai peninsula on 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 so-called Islamic State militant group said it had smuggled an explosive device on board.
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.
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.
In October 1999, the first officer of a Boeing 767 deliberately crashed the plane into the Atlantic Ocean about 60 miles south of Nantucket Island, Massachusetts, killing all 217 people on board.
