The bodies of some of the victims of a plane crash in Egypt have arrived in St Petersburg on a Russian cargo plane.

The Metrojet Airbus A321-200 crashed in the Sinai Peninsula on Saturday, 23 minutes after taking off from the Egyptian Red Sea resort of Sharm el-Sheikh en route to St Petersburg.

Russian officials said the plane broke up at high altitude, scattering fragments of wreckage and bodies over a wide area in Sinai.

All 224 people on board died, all but five of them Russians.

Only an external impact could have caused the crash of a Russian plane in Egypt that killed all 224 people on board, a top airline official has said.

"We rule out a technical fault of the plane or a pilot error," said Alexander Smirnov, deputy general director of Metrojet. "The only possible explanation could be an external impact on the plane."

But when pressed for more details about what type of impact and what could have caused it, Mr Smirnov insisted that he was not at liberty to discuss details because the investigation was ongoing.

Viktor Yung, another deputy director general of Metrojet, said the crew did not send a distress call and they did not contact traffic controllers before the crash.

An Egyptian official had previously said the pilot radioed that the plane was experiencing technical problems and he intended to try to land at the nearest airport.

The Russian government plane carrying the bodies of 140 passengers touched down at St Petersburg's Pulkovo airport in the early hours.

Emergency Situations Minister Vladimir Puchkov said in a televised news conference that another plane with more crash victims will travel from Cairo to St Petersburg later on Monday.

Mr Puchkov said the search for bodies at the Sinai crash site should conclude by 10pm local time.

President Vladimir Putin declared a nationwide day of mourning on Sunday, and flags flew at half-mast across the country. St Petersburg, where many of the victims are from, is holding three days of mourning.

Hundreds of mourners in Russia's second-largest city took flowers, pictures of the victims, stuffed animals and paper planes to the city's airport on Sunday. Others went to churches and lit candles in memory of the dead.

In Sinai, aviation experts and search teams have been combing an area of more than six square miles (16 sq km) to find bodies and pieces of the jet. The Egyptian government said that by midday on Sunday163 bodies had been recovered.

Alexander Neradko, head of Russia's federal aviation agency, told reporters on Sunday that the large area over which debris fragments were found indicates that the plane disintegrated while flying at high altitude. He would not comment on any possible reason for the crash, citing the ongoing investigation.

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

A local affiliate of the extremist Islamic State group claimed it brought down the aircraft, which crashed in northern Sinai where the Egyptian military and security forces have battled militants for years. Russian officials have dismissed that claim.

When planes do break up in mid-air it is usually because of one of three factors - a catastrophic weather event, a mid-air collision or an external threat, such as a bomb or a missile.

With no indication that those events played a role in the crash, Todd Curtis, a former safety engineer with Boeing, said investigators will be looking at more unusual events, such as an on-board fire or corrosion that caused a structural failure.

The flight recorders will provide key information, including the plane's airspeed and whether it was on autopilot.

Alexander Smirnov, Metrojet's deputy director, described the A321 as a reliable aircraft which would not fall into a spin even if the pilots made a grave error because automatic systems correct crew mistakes.

Egyptian President Abdel-Fattah el-Sissi cautioned that the cause of the crash may not be known for months.

"It's very important that this issue is left alone and its causes are not speculated on," he told top government officials, including members of the military and security forces. The investigation "will take a long time" and "needs very advanced technologies".