Intelligence suggests the Russian plane that crashed in Egypt killing all 224 people on board may have been brought down by a bomb, US and UK officials say.
But they say they have yet to reach a formal conclusion.
Britain earlier suspended flights to and from the resort of Sharm el-Sheikh, from where the flight had departed.
Egypt has dismissed claims by militants linked to Islamic State (IS) that they brought down the plane.
Russian experts say it is too early to say.
Separately on Wednesday, Egyptian officials said the cockpit voice recorder of the Metrojet Airbus 321 was badly damaged in the crash.
However, they managed to extract information from the flight data recorder which is ready to be analysed by investigators, Egypt's Civil Aviation Minister Mohamed Hossam Kamal said. Theories about causes What we know so far Who were the victims?
'Significant possibility'
The plane, bound for St Petersburg, crashed in Egypt's Sinai desert just 23 minutes after take-off from Sharm el-Sheikh on Saturday.
Most of those on board the plane, which was operated by Russian airline Kogalymavia, were Russian.
Media captionDid bomb cause Sinai plane crash?"We have concluded that there is a significant possibility that the crash was caused by an explosive device on board the aircraft," UK Foreign Secretary Philip Hammond told reporters after the government's crisis response committee met late on Wednesday.
He said Britain was suspending all flights to and from Sharm el-Sheikh indefinitely and UK nationals already there would be helped to leave once extra security measures were put in place.
Earlier, Prime Minister David Cameron's office said UK aviation experts had been sent to Sharm el-Sheikh to assess the security situation there.
Egyptian Foreign Minister Sameh Shoukry called the UK response premature and unwarranted, telling the BBC that Egypt had taken "exceptional measures" to enhance security at Sharm el-Sheikh airport.
The UK government is currently hosting Egyptian President Abdul Fattah al-Sisi. Image copyright AFPImage caption Investigators hope flight data will provide clues as to what caused the plane to crash An unnamed US official told the Associated Press news agency they had reached the "tentative conclusion", after intercepting communications, that an IS affiliate in the Sinai peninsula had planted an explosive device on the plane.
"A bomb is a highly possible scenario," another US official told the AFP news agency.
But the officials stressed that forensic evidence, including the flight recorders, was still being analysed.
On Tuesday, US media reported that a military satellite had detected a "heat flash" over the Sinai at the time of the crash.
It said data was still being analysed and that although the flash could have been caused by a bomb, a fuel tank or engine explosion was also possible.
Mr Sisi, in a BBC interview on Tuesday, dismissed as "propaganda" claims that militants linked to IS brought down the aircraft.
The militants have carried out a series of deadly attacks against Egyptian security forces in recent years.
Kogalymavia earlier this week blamed "external influence" for the crash, but the head of Russia's aviation agency said such talk was premature.
Timeline: the course of flight KGL9268
05:58 Egyptian time (03:58 GMT): Flight leaves Sharm el-Sheikh 06:14 Egyptian time (04:14 GMT): Plane fails to make scheduled contact with air traffic control based in Larnaca, Cyprus, according to an official with Russia's air transport agency 06:17 Egyptian time, approx (04:17 GMT): Plane comes down over the Sinai peninsula 11:12 Egyptian time (09:12 GMT): Flight had been due to land in St Petersburg's Pulkovo airport
Egyptian President Abdul Fattah al-Sisi has described claims that militants linked to Islamic State (IS) brought down a Russian airliner as "propaganda".
He told the BBC that it was too early to say what had caused the crash.
The plane broke up over the Sinai peninsula on Saturday, killing all 224 people on board.
On Monday, the airline Kogalymavia blamed "external influence" for the crash.
But the head of Russia's Federal Aviation Agency, Aleksandr Neradko, told Russian TV that such talk was premature and "not based on any proper facts".
Media captionPresident Sisi said the area of Sinai concerned was "under our full control"President Sisi also warned against jumping to conclusions, telling the BBC: "All those interested in the matter are welcome to participate in the investigation.
"When there is propaganda that it crashed because of Isis (IS), this is one way to damage the stability and security of Egypt and the image of Egypt. Believe me, the situation in Sinai - especially in this limited area - is under our full control."
The US director of national intelligence, James Clapper, said there was no "direct evidence of any terrorist involvement yet" adding: "It's unlikely, but I wouldn't rule it out."
The IS affiliate known as Sinai Province issued a written statement and an audio message saying that it brought down the passenger jet.
The group, which pledged allegiance to IS a year ago, is thought to be aiming to take control of the Sinai Peninsula in order to turn it into an Islamist province.
However, experts doubt it possesses the weapons capable of bringing down an airliner. Image copyrightAPImage caption Debris is spread over a wide area The Airbus 321 had been carrying 217 passengers, including 25 children, and seven crew members. Most of those on board were Russians.
It crashed 23 minutes after taking off from the Egyptian Red Sea resort of Sharm el-Sheikh en route to St Petersburg.
The plane's flight recorders have been found and sent for analysis.
Debris was spread across a wide area of Sinai, officials said.
At a news conference in Moscow on Monday, the deputy director of Kogalymavia which owns the Metrojet fleet of aircraft, ruled out a technical fault or pilot error. "The only [explanation] for the plane to have been destroyed in mid-air can be specific impact, purely mechanical, physical influence on the aircraft," Alexander Smirnov said.
"There is no such combination of failures of systems which could have led to the plane disintegrating in the air," he added.
He said the plane lost speed and started descending rapidly, and the crew made no attempt to get in contact and report about the situation on board.
The US broadcaster CBS reported that a US satellite had detected a "heat flash" over the Sinai at the time the Russian plane crashed.
It said the data was still being analysed and although the flash could have been caused by a bomb, it was also possible that a mechanical failure had caused an explosion in a fuel tank or the engine.
So far, the bodies of 144 of those killed in the crash have been flown back to St Petersburg.
Russia is due to conclude three days of official mourning on Tuesday.
Timeline: the course of flight KGL9268
05:58 Egyptian time (03:58 GMT): Flight leaves Sharm el-Sheikh, a statement from the Egyptian cabinet says 06:14 Egyptian time (04:14 GMT): Plane fails to make scheduled contact with air traffic control based in Larnaca, Cyprus, according to Sergei Izdolsky, an official with Russia's air transport agency 06:17 Egyptian time, approx (04:17 GMT): Plane comes down over the Sinai peninsula, according to Airbus 11:12 Egyptian time (09:12 GMT): Flight had been due to land in St Petersburg's Pulkovo airport
The Russian airline Kogalymavia has blamed "external influence" for Saturday's Sinai plane crash which killed 224 people.
A senior airline official said: "The only reasonable explanation is that it was [due to] external influence."
An investigation by aviation experts using data from the aircraft's "black boxes" has yet to give its conclusions.
The head of Russia's Federal Aviation Agency said it was premature to speculate on the cause of the crash.
"This kind of talk is... not based on any proper facts," Aleksandr Neradko said on Russian TV.
The Kremlin has also warned against speculation as to the possible causes of the crash.
James Clapper, the US director of national intelligence, said there was no "direct evidence of any terrorist involvement yet" adding: "It's unlikely, but I wouldn't rule it out."
Russian President Vladimir Putin has described the crash as an "enormous tragedy" and expressed his condolences to the families of the victims.
"Without any doubt, everything must be done to create an objective picture of events so that we know what happened and can react accordingly," he said.
At a news conference in Moscow, the deputy director of the airline, which was later renamed Metrojet, ruled out a technical fault and pilot error.
"The only [explanation] for the plane to have been destroyed in mid-air can be specific impact, purely mechanical, physical influence on the aircraft," Alexander Smirnov said.
"There is no such combination of failures of systems which could have led to the plane disintegrating in the air," he added. Another airline official acknowledged that there had been previous damage to the plane's tail in 2001 during take-off.
But he said that the damage had been repaired, and was not thought to be a factor in the crash.
However, the widow of the plane's co-pilot told Russian TV her husband had complained about the aircraft's technical condition.
Media captionSarah Rainsford reports from a morgue in St Petersburg where bodies are being identifiedThe Airbus 321 lost speed and started descending rapidly, and the crew made no attempt to get in contact and report about the situation on board, Mr Smirnov added.
The bodies of 144 of those killed in the crash have been flown back to the Russian city of St Petersburg, where the plane had been headed from the Egyptian resort of Sharm el-Sheikh.
A second flight with more bodies is expected to arrive in St Petersburg on Monday evening.
Russian Emergency Situations Minister Vladimir Puchkov said that searches in a 20-sq-km (7.7-sq-Mile) area around the crash site had ended and that a 30-sq-km area was now being searched.
Mr Puchkov said searches would not end until all the bodies have been recovered, AP reports.
The plane was carrying 217 passengers, including 25 children, and seven crew members - mostly Russians.
Timeline: the course of flight KGL9268
Image copyrightEPA05:58 Egyptian time (03:58 GMT): Flight leaves Sharm el-Sheikh, a statement from the Egyptian cabinet says 06:14 Egyptian time (04:14 GMT): Plane fails to make scheduled contact with air traffic control based in Larnaca, Cyprus, according to Sergei Izdolsky, an official with Russia's air transport agency 06:17 Egyptian time, approx (04:17 GMT): Plane comes down over the Sinai peninsula, according to Airbus 11:12 Egyptian time (09:12 GMT): Flight had been due to land in St Petersburg's Pulkovo airport
Image copyright AFP 
