The bodies of 140 victims of the Metrojet plane crash have arrived in St Petersburg, where most of the passengers were from, as Russian officials confirmed the plane broke up at high altitude.

All 224 people on board the Russian airliner were killed on Saturday when it crash-landed shortly after take-off from Egypt’s Sharm el-Sheik resort.

The plane had been en route to St Petersburg when it crashed 23 minutes into its journey, scattering bodies and fragments of the planes over a wide area in the Sinai Province.

A 16 square kilometre area in Sinai has been combed by search teams, who had recovered 163 bodies by midday Sunday. More victims’ bodies are expected to be flown to Russia on Monday night.

Forensic experts have begun identifying the victims’ bodies, according to a Russian official, many of which were in pieces, according to an unnamed Egyptian morgue worker.

Russia observed a day of mourning on Sunday (AP)

“Maybe 75 of the ones we have are full corpses," he said.

"Some are burned, others have blunt trauma wounds and fractured skulls. The others were mainly arms and legs. It could be that these parts belong to some of the corpses, but we don’t know without DNA analysis."

While it has been confirmed the plane broke up at high altitude, Egyptian President Abdel-Fattah el-Sissi cautioned that the cause of the crash may not be known for months. Both of the plane’s black boxes have been recovered and are now to be analysed, though a number of theories on how and why the plane crashed have already been raised.

Isis-affiliated group fired anti-aircraft missile

Egypt: Russian A321 plane likely fell apart in the air over Sinai - Rosaviation head

Wilayat Sinai, an Isis-linked group operating in the area where the plane crashed, has attempted to claim responsibility for the tragedy on behalf of the terrorist group “in response to Russian airstrikes that killed hundreds of Muslims on Syrian land”.

Reports suggested Isis claimed the plane was shot down by an anti-aircraft missile, while supporters of the militant group shared a video claiming to show the plane exploding mid-air before falling to the ground while trailing smoke.

But authorities have refuted claims the plane was brought down in a terrorist attack. Russia’s transport minister Maksim Sokolov said suggestions a missile had downed the plane “can’t be considered accurate,” while Egypt’s army spokesperson Mohamid Samir told The Guardian there is “no proof” that terrorists were responsible for the crash.

Russian passenger plane crashes in Egypt Show all 20 1 /20 Russian passenger plane crashes in Egypt Russian passenger plane crashes in Egypt Egyptian soldiers collect personal belongings of plane crash victims at the crash site of a passenger plane bound for St. Petersburg in Russia that crashed in Hassana, Egypt's Sinai Peninsula Russian passenger plane crashes in Egypt Egyptian soldiers collect personal belongings of plane crash victims at the crash site of a passenger plane bound for St. Petersburg in Russia that crashed in Hassana, Egypt's Sinai Peninsula Russian passenger plane crashes in Egypt In this Russian Emergency Situations Ministry photo, made available on Monday, Nov. 2, 2015, showing Metrojet Airbus A321-200 flight 7K9268 flight recorder on display at an undisclosed location in Egypt Russian passenger plane crashes in Egypt Mourners lay flowers at Pulkovo International Airport outside St. Petersburg. Russia on 1 November mourned its biggest ever air disaster after a passenger jet full of Russian tourists crashed in Egypt's Sinai, killing all 224 people on board. Flags were at half mast on the parliament building, in the Kremlin, and on other official buildings in honour of the victims, most of whom were from Russia's second-largest city of Saint Petersburg Russian passenger plane crashes in Egypt People pay their respects at the entrance of Pulkovo airport outside St. Petersburg, during a day of national mourning for the plane crash victims Russian passenger plane crashes in Egypt Debris from the plane crash in Egypt Russian passenger plane crashes in Egypt A piece of an engine of Russian MetroJet Airbus A321 at the site of the crash in Sinai, Russian passenger plane crashes in Egypt The crash site debris Flight 7K9268 crashed in the Sinai peninsula, in all probability killing every one of the 224 people on board AFP/Getty Russian passenger plane crashes in Egypt The crash site debris Debris lies strewn across the sand at the crash site EPA Russian passenger plane crashes in Egypt Relatives in St Petersburg Relatives react after a Russian airliner with 217 passengers and seven crew aboard crashed, as people gather at the Kogalymaviaís information desk at Pulkovo airport in St Petersburg on 31 October AP Russian passenger plane crashes in Egypt Relatives in St Petersburg A relative of a passenger of MetroJet Airbus A321 at Pulkovo II international airport in St Petersburg, Russia, 31 October 2015. EPA Russian passenger plane crashes in Egypt The plane's journey The plane's last recorded radar position above the northern Sinai peninsula Flightradar24 Russian passenger plane crashes in Egypt Where it crashed A satellite view from Google Maps of the rough area where the plane crashed, in the mountainous Hassana region of the Sinai peninsula. Google Maps Russian passenger plane crashes in Egypt The plane The Metrojet's Airbus A-321 with registration number EI-ETJ that crashed in Egypt's Sinai peninsula REUTERS/Kim Philipp Piskol Russian passenger plane crashes in Egypt The plane The crashed Airbus A321 at Domodedovo international airport, outside Moscow,, on 20 October Russian passenger plane crashes in Egypt Relatives at in St Petersburg A relative of a passenger on MetroJet Airbus A321 at Pulkovo II international airport in St Petersburg EPA Russian passenger plane crashes in Egypt Relatives at in St Petersburg Relatives of passengers of MetroJet Airbus A321 at the Crown Plaza hotel in St Petersburg EPA Russian passenger plane crashes in Egypt Bodies being repatriated An Egyptian soldier prays as emergency workers prepare to unload bodies of victims from a police helicopter to ambulances at Kabrit military airport on 31 October. AP Russian passenger plane crashes in Egypt Bodies being repatriated Ambulances line up as emergency workers unload bodies at Kabrit military airport, 20 miles north of Suez, on Saturday AP Russian passenger plane crashes in Egypt Bodies being repatriated Egyptian paramedics load the corpses of victims into a military plane at Kabrit military air base by the Suez Canal on October 31, 2015 AFP/Getty Images

“We will know the true reasons [of the crash] when the civil aviation authority in coordination with the Russian authorities completes its investigation," he said.

"But the army sees no authenticity to their claims or their video."

The plane was also cruising at 31,000ft before any incident occurred, which experts have said is out of range for Wilayat Sinai’s air-defence systems, known as Manpads, meaning the plane was too high to have been hit.

Bomb on board

Russian officials have said the plane likely broke up mid-air, appearing to have split in two, with one part burning up and another crashing into a rock, but have stressed it is too early to draw conclusions from this.

A former safety engineer with Boeing says that when planes break up in mid-air, it is usually due to one of three factors: a catastrophic weather event, a collision or an external threat, such as a bomb or a missile.

Todd Curtis says if there are no indications such events played a role in the crash, investigators will be looking at more unusual events, such as an on-board fire or corrosion that caused a structural failure.

But Professor Michael Clarke at the Royal United Services Institute think-tank told the BBC that reports of the aircraft splitting in two suggest a “catastrophic failure, not a mechanical failure,” which could mean an explosion on board.

“If we have to guess at this stage, it’s much more likely to have been a bomb on board rather than a missile fired from the ground," he said.

Technical failure on board

Reports that the plane was in a poor condition have circulated as a possible reason for the crash. A source at Sharm el-Sheikh airport who spoke to Russian news agency Ria Novosti claimed the plane had experienced “engine start failures several times over the past week" which the crew had apparently flagged to airport technicians.

The co-pilot himself had reportedly complained to his wife about the technical condition of the airliner before take-off, according to an interview on Russian state-controlled TV.

Natalya Trukacheva, wife of the late co-pilot Sergei Trukachev, told NTV: “He complained before the flight that the technical condition of the aircraft left much to be desired”.

The pilot was said to have planned an emergency landing, radioing in to air traffic control to report technical difficulties on board, an official from the Aviation Incident Committee said, shortly before contact was lost.

In addition, the Aviation Safety Network reported the aircraft had in 2001 suffered a ‘tail strike’, which occurs when the tail of an aircraft hits the runway, which took three months to repair. But the aircraft received a certificate of airworthiness earlier this year, regulators in Ireland, where it was registered, said on Monday.

Egypt’s minister for civil aviation, Hossam Kamal, has since said this was not true.

“Up until the crash happened, we were never informed of any faults on the plane, nor did we receive any SOS calls,” he told a press conference. He added that pre-flight checks and all-contact with air traffic control has been normal.