Egypt has said terrorism was more likely than a technical fault to have caused EgyptAir flight MS804 to swerve abruptly and fall out of the night sky on Thursday, plunging into the Mediterranean with 66 people onboard.

The reason for the catastrophe -- which occurred on a flight from Paris to Cairo, in perfect weather conditions and soon after the plane entered Egyptian airspace -- remained unclear on Thursday night.

France and Egypt said no theory could yet be ruled out. "The information we have gathered confirms, alas, that this plane has crashed, and it has disappeared," the French president, François Hollande, said. "We have a duty to know everything about the cause and what has happened," he added. "No theory is ruled out and none is certain right now."

Egypt's aviation minister, Sherif Fathi, said he did not want to prematurely draw conclusions, but added: "The possibility of having a different action or a terror attack, is higher than the possibility of having a technical failure."

Debris spotted bobbing in the sea south of the Greek island of Karpathos was initially thought to be from the crash. On Thursday afternoon, a Greek frigate noticed two plastic objects floating about 230 miles south-east of the island of Crete. The ship's captain, Tarek Wahba, described them as a "lifejacket and a chair". He posted the images on Facebook. Some of the debris was "blue and white", the colour of the airline's livery.

EgyptAir tweeted that it was wreckage from the plane but later retracted the claims. "We stand corrected on finding the wreckage because what we identified is not a part of our plane," the airline's vice-president, Ahmed Adel, told CNN.

Egypt's president, Abdel Fatah al-Sisi, ordered the civil aviation ministry, the army's search and rescue centre, the navy, and air force to take all necessary measures to locate debris from the aircraft. In a statement issued by his office, Sisi also ordered an investigative committee formed by the civil aviation ministry to immediately start investigating the causes of the plane's disappearance.

Security at the various airports that the missing plane had visited in the preceding 24 hours, including Paris's Roissy-Charles de Gaulle, is likely to come under close scrutiny.

Under the state of emergency imposed in France since the November terror attacks, the French capital's major airport has seen security measures ramped up. As well as making repeated stops at Cairo International airport, the plane had also visited Asmara in Eritrea and Tunis in Tunisia.

The Airbus A320 went down about 130 miles from Karpathos, Greek aviation sources indicated. C-130 aircraft and at least eight ships spent the day hunting for wreckage. Its experienced captain -- he had clocked up 6,275 flying hours, according to EgyptAir -- did not send a distress signal.

The airline said in a statement: "EgyptAir sincerely conveys its deepest sorrow to the families and friends of the passengers on board flight MS804. Family members of passengers and crew have been already informed and we extend our deepest sympathies to those affected."

Hollande said: "When we have the truth we will draw our conclusions; whether this was an accident or something else, perhaps terrorist. We will have the truth."

The plane took off from Charles de Gaulle airport at 11"09 p.m. local time on Wednesday night, bound for Cairo. Contact was lost at about 2:30 a.m. Egypt local time when it abruptly disappeared.

The Greek defence minister, Panos Kammenos, said the plane made "sudden swerves" in mid-air and plunged before dropping off radars in the southern Mediterranean.

"The plane carried out a 90-degree turn to the left and a 360-degree turn to the right, falling from 37,000ft to 15,000ft, and the signal was lost at around 10,000ft," Kammenos said. Controllers had tried to make contact with the plane 10 miles before it exited Greek airspace, the aviation authority said. They got no response.

Egypt's prime minister, Sherif Ismail, said terrorism was one possible explanation. "We cannot exclude anything at this time or confirm anything. All the search operations must be concluded so we can know the cause," Ismail said.

Greek authorities were investigating an account from the captain of a merchant ship who reported seeing a "flame in the sky" about 130 nautical miles south of Karpathos. The plane was about 10 miles inside Egyptian airspace when it vanished.

Officials from multiple U.S. agencies told Reuters that a review of satellite imagery so far had not produced any signs of an explosion aboard the EgyptAir flight. The anonymous sources said the conclusion was the result of a preliminary examination of imagery. They said the US had not ruled out any possible causes for the crash, including mechanical failure, terrorism or a deliberate act by the pilot or crew.

The plane's unexplained crash follows serious concerns about security at Egypt's airports. Some 224 people were killed on October 31 last year after a bomb was smuggled on to a Russian passenger jet. The plane, which took off from the resort of Sharm el-Sheikh, was brought down over the Sinai peninsula.

Britain warned Egyptian officials about lax security at the airport back in 2014. Egypt initially denied any terrorist link. The Kremlin later said an explosive device was responsible and a local branch of the extremist group Islamic State claimed responsibility within hours.

On Thursday, however, there was no Isis claim of responsibility, with the group's usual media channels silent. Russia's spy chief Alexander Bortnikov, however, said it was "in all likelihood a terror attack".

In recent years Egypt has faced a growing threat from Isis-affiliated groups, including Wilayat Sinai, based in the Sinai peninsula. It has claimed several bombings and shootings in the Nile valley against a backdrop of state repression under the Egyptian president, Abdel-Fatah al-Sisi.

Those on board the missing flight MS804 included 56 passengers and 10 crew, including three security guards. There were 30 Egyptians, 15 French nationals, two Iraqis and one person each from the UK, Belgium, Kuwait, Saudi Arabia, Sudan, Chad, Portugal, Algeria and Canada. At least two babies and one child were on board, the airline said. The Briton was named as Richard Osman, a 40-year-old geologist who had recently become a father for the second time.

Those aboard also included a Kuwaiti economics professor, Abdulmohsen al-Muteiri, Kuwait's foreign ministry confirmed. A father of two, he was heading to Cairo for a three-day conference. Another was a young military student from Chad who was flying home to visit his mother.

Some of the crew have been named: the captain, Mohamed Said Shoukair; the first officer, Mohamed Mamdouh Ahmed; and Mirvat Zaharia Zaki Mohamed, the head flight attendant.

Retrieving the plane's black box is likely to be a long and fraught operation. The head of Greece's air traffic control board, Serafeim Petrou, told the Guardian it was a "fact the plane had crashed", adding: "Most probably, and very unfortunately, it is at the bottom of the sea."

Petrou said tracing the cause and retrieving wreckage would therefore take time. "Nothing can be excluded. An explosion could be a possibility but, then, so could damage to the fuselage," he said.

Egypt was leading international search efforts, backed by France, Greece, Turkey, and the U.S. President Obama said the U.S. navy had dispatched a P-3 Orion long-range aircraft from a base in Sicily.

France's prime minister, Manuel Valls, offered to send planes and boats to help. He also said that "no theory can be ruled out" in investigating MS804's disappearance.

Jean-Paul Troadec, the former president of the French air investigation bureau, told Europe 1: "We can make certain hypotheses. There's a strong possibility of an explosion on board from a bomb or a suicide bomber. The idea of a technical accident when weather conditions were good seems possible but also not that likely."

Another possibility, he suggested, was that the plane might have been shot down - the fate of Malaysian Airlines MH17, downed in July 2014 above rebel-held eastern Ukraine by a sophisticated Russian surface-to-air missile.

"If the crew didn't send an alert signal, it's because what happened was very sudden," Troadec said.

EgyptAir warned against "misleading" speculation. The airline tweeted:

of the disappearance of EGYPTAIR flight MS804 and the company confirms that the reason of disappearance hasn't been yet confirmed.

- EGYPTAIR (@EGYPTAIR) May 19, 2016

Relatives of some of the passengers gathered at Cairo International airport.

"There's no information inside. They're not telling us anything for sure," said one young woman, who did not disclose her name. She said she had come to the arrival hall in the hope of hearing news of her friend Samar, one of the 30 Egyptian passengers on board the missing flight.

The UK foreign secretary, Philip Hammond, had earlier confirmed that a British passport holder was on board, and the Foreign Office in London said it was working closely with the Egyptian and French authorities.

Deeply concerned by missing #EgyptAir. Can confirm British passport holder was on board & FCO is supporting the missing passenger's family.

- Philip Hammond (@PHammondMP) May 19, 2016

An EgyptAir plane was hijacked and diverted to Cyprus in March. A man who admitted to the hijacking and is described by Cypriot authorities as "psychologically unstable" is in custody in Cyprus.

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