A Russian airliner which crashed in Egypt's Sinai peninsula, killing all 224 people on board, broke up in mid-air, a Russian official says.

Victor Sorochenko, the head of Russia's Interstate Aviation Committee, said it was too early to conclude what caused the crash on Saturday.

He told reporters debris was found across a 20sq km-wide area of Sinai. So far 163 bodies have been found.

Russia observed a day of mourning on Sunday after its worst air disaster.

Sorochenko spoke after visiting the site of the debris, near the village of Hasana. The head of Russia's Air Transport Agency, Aleksandr Neradko, said that "all signs attest to the fact that the aircraft disintegrated in the air at a high altitude".

Jihadists allied to so-called Islamic State in Sinai,where such groups are active,had made a claim on social media that they had brought down flight KGL9268.

But Egyptian Prime Minister Sharif Ismail said experts had confirmed that a plane could not be downed at 9,450m (31,000ft), the altitude the Airbus 321 was flying at, by weapons the militants are known to possess.

President Abdul Fattah al-Sisi also urged caution on Sunday, saying the investigation into the cause was a "complicated matter" that could take months.

Russian Transport Minister Maksim Sokolov said no evidence had been seen that indicated the plane was targeted, and IS has not produced pictures or video footage to substantiate its claim.

Analysis by BBC Monitoring found that a video purporting to show the downing of a plane did not appear to be an official IS clip, and was not shared on official IS channels.

A number of major airlines - Emirates, Air France-KLM, Lufthansa, and Qatar Airways - have decided not to fly over the Sinai Peninsula until more information is available. Two smaller carriers, flydubai and Air Arabia, also said they would re-route flights, while Etihad Airways said it would avoid only "certain areas of airspace" over Sinai.

Germany's transport ministry has told German airlines not to follow the same route taken by the Russian plane.

British Airways said it regularly assessed the safety of its routes. BA said it would not confirm flight routes, but that it "would never fly a route unless it was safe to do so".

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

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

Egyptian officials said some bodies had been recovered within a radius of 5km on Saturday, but that of a three-year-old girl was found 8km from the scene.

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

The plane's flight recorders have been found and sent for analysis, officials said.

Russia's transport regulator said on Sunday it would check all the airline's A-321 planes, but Kogalymavia said this would not affect their operations.

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 ground service official who carried out a pre-flight inspection of the plane said the aircraft appeared to be in good shape.

"We are all shocked. It was a good plane. Everything checked out in 35 minutes," the official told AP.

However, the widow of the plane's co-pilot told Russian TV her husband had complained about the aircraft's technical condition.

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

Most of the passengers were Russian, but at least three were Ukrainian and one Belarusian.

A plane returning 162 bodies to Russia left Cairo late on Sunday, an airport official said.

Timeline: the course of flight KGL9268

05:58 Egyptian time (03:58 GMT): Flight leaves Sharm el-Sheikh, the Egyptian cabinet says in a statement .

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