An investigation is under way after a Russian airliner crashed in Sinai, killing all 224 people on board.

Egypt's prime minister said a technical fault was the most likely cause, dismissing claims from Islamic State militants that they were responsible.

However, three airlines - Emirates, Air France and Lufthansa - have decided not to fly over the Sinai Peninsula until more information is available.

Russia is observing a day of mourning after its worst air disaster.

The plane's black boxes have been found and sent for analysis, officials said.

The BBC's Sally Nabil in Cairo says the crash has been a major blow to Egypt's already struggling tourism industry, and the Egyptian authorities are trying very hard to accelerate the investigation process.

The Kogalymavia Airbus A-321 came down early on Saturday, shortly after leaving the resort of Sharm el-Sheikh for the Russian city of St Petersburg.

Egypt's civil aviation minister Hossam Kamal said there had been no sign of any problems on board the flight, contradicting earlier reports that the pilot had asked to make an emergency landing.

An Egyptian official had previously said that before the plane lost contact with air traffic controllers, the pilot had said the aircraft was experiencing technical problems and he intended to try to land at the nearest airport.

In Russia, the wife of the plane's co-pilot, Sergei Trukhachev, said her husband had complained about the plane's condition.

Natalya Trukhacheva told state-controlled NTV that their daughter had called her father before the flight left Sharm el-Sheikh.

"He complained before the flight that the technical condition of the aircraft left much to be desired," she said.

One Egyptian official said the plane suddenly "disappeared off the radar"

Prime Minister Sharif Ismail (third from right) visited the site of the crash on Saturday afternoon

Russian and French investigators have joined the Egyptian-led probe, along with experts from Airbus, which is headquartered in France.

Meanwhile a Russian team has arrived at the crash site to join efforts to recover bodies, Russian media say.

A criminal case had been opened against Kogalymavia for "violation of rules of flight and preparation for them", Russia's Ria news agency reported.

Police have searched the company's offices.

Kogalymavia spokeswoman Oksana Golovina insisted the 18-year-old plane was "fully, 100% airworthy" and added that the pilot had 12,000 hours of flying experience.

IS claims dismissed

In Sinai itself, where jihadist groups are active, militants allied to IS made a claim on social media that they brought down flight KGL9268.

But Egyptian Prime Minister Sharif Ismail dismissed the claim, saying experts had confirmed that a plane could not be downed at the altitude the Airbus 321 was flying at.

ussian Transport Minister Maksim Sokolov told Interfax news agency that "such reports cannot be considered true". No evidence had been seen that indicated the plane was targeted, he said.

Egypt's civilian aviation ministry said the plane had been at an altitude of 9,450m (31,000ft) when it disappeared.

Security experts say a plane flying at that altitude would be beyond the range of a shoulder-launched surface-to-air missile (Manpad), which Sinai militants are known to possess.

However, German carrier Lufthansa said it would avoid flying over the Sinai peninsula "as long as the cause for today's crash has not been clarified". On Saturday evening, Air France-KLM and Emirates said they were following suit.

British Airways and easyJet said their routes were regularly reviewed, but that they had no plans to alter their routes to and from Sharm el-Sheikh.

A hotel near St Petersburg airport has become a gathering point for relatives of those on flight 9268. There are medics here to help them, and we saw an Orthodox priest. Some have been giving DNA samples, to try to identify their loved ones.

In the lobby, men from the investigative committee are huddled over files and phones - beside cabinets full of souvenirs. For some reason, riot police are patrolling the corridors too.

Occasionally, someone emerges red-eyed from the room where officials are passing on what information and answers they can to relatives. But it's still impossible to say why this happened.

A spokeswoman for the airline says the pilot was experienced, the Airbus plane had been through all its safety checks. But there are a lot of people here with urgent questions about why their friends and relatives are not safely home tonight after a holiday in the Egyptian sun.

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The plane was carrying 217 passengers, including 25 children, Russian transport authorities said. There were seven crew members on board.

Egyptian officials had said 213 of the passengers were Russian and four were Ukrainian, but Russian officials said at least one of the victims was from Belarus.

The bodies of 163 victims have so far been recovered and taken to Cairo.

The first bodies to be returned to Russia are expected to be flown to St Petersburg on Sunday.

One unnamed official described a "tragic scene" with bodies of victims still strapped to seats.

The plane appeared to have split in two, he told Reuters, with one part burning up and the other crashing into a rock.

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Timeline: the course of flight KGL9268