AFP/ Saint Petersburg

A plane carrying the remains of 144 people killed when a Russian airliner crashed in Egypt landed in Saint Petersburg early on Monday, hours after investigators probing the doomed flight said it had broken up in the air.

The emergency Russian plane, the first to bring back the bodies of those who died when the A-321 plummeted from the sky above the restive Sinai Peninsula, arrived in the Pulkovo airport of Russia's second-largest city.

Investigators from several countries have joined an Egypt-led probe to determine what brought down Russian airline Kogalymavia's flight 9268 on Saturday, killing all 224 people on board, en route from Sharm el-Sheikh to Saint Petersburg.

They have recovered the "black box" flight recorders of the Airbus, and the head of an Irish mission that will join the Egypt-led probe into the disaster said the results from the recorders should be ready in a few days.

The head of the Russian air transport agency Alexander Neradko on Sunday said it appeared the aircraft disintegrated while flying at high altitude, echoing similar comments from other top aviation officials.

"All signs prove that the structure of the plane disintegrated in the air at a high altitude," he told Russian state television.

The bodies of those sent back to Saint Petersburg were due to be taken in a motorcade to a crematorium for identification, which will begin later on Monday, according to Russia's emergency ministry.

Russian officials confirmed that 144 bodies were on board the plane that landed at Saint Petersburg, after the worst aviation disaster in the country's history.

Egypt had earlier said the remains of 162 people would be flown back, out of 214 Russian passengers said to have been on the flight, along with three Ukrainians and seven crew.

Family members have been providing DNA samples at a crisis centre set up close to the Russian airport, now the site of an impromptu memorial where people have brought flowers and cuddly toys to commemorate the victims, many of them children.

- Investigations widen -

Flags flew at half mast in Russia on Sunday on a national day of mourning for the victims, and thousands gathered in Saint Petersburg's Palace Square to observe a minute's silence and release doves and balloons into the darkening sky.

"It was impossible for me not to come," said Nika Kletskikh, 27, who lost a friend in the crash. "It's so awful to think that she's no longer there."

The crash site in the Wadi al-Zolomat area of North Sinai was littered with blackened aircraft parts Sunday as the smell of burnt metal lingered, an AFP correspondent said.

Soldiers guarded dozens of bags and suitcases belonging to passengers from flight KGL 9268 --- a tiny red jacket among the recovered items, underlining the horror of the tragedy that killed 17 children.

Officers involved in the search efforts said rescue crews had recovered 168 bodies so far, including one of a girl found eight kilometres (five miles) from the main wreckage.

Both Cairo and Moscow have downplayed the claim from Egypt's IS branch that it brought down the aircraft flown by the airline Kogalymavia, operating under the name Metrojet.

Investigators from Egypt, Russia, France and Airbus are now looking at other possible causes, and a Russian team including Sokolov and the emergencies minister, Vladimir Puchkov, have visited the scene.

Jurgen Whyte, chief inspector with Ireland's Air Accident Investigation Unit (AAIU) who is leading a three-person team due to join the probe on Monday, said readings from the black boxes would direct the course of the investigation.

"Nothing can be said until we have access to the recorders, and luckily they have been recovered," he told AFP.

- Plane passed safety review -

On Sunday, the AAIU said it had given the A-321 a clean bill of health earlier this year after its annual review, which was carried out in Ireland, where the aircraft was registered.

Russia has a dismal air safety record, and while larger carriers have begun upgrading ageing fleets, the crash is likely to raise concerns about smaller airlines such as Kogalymavia.

On Sunday, Russia's transportation watchdog ordered Kogalymavia to perform a full check on its A-321s, although the airline denied this was a de facto grounding of its other six aircraft of the same model.

Experts have dismissed claims from an IS-affiliated insurgency group in the Sinai that it brought down the aircraft in revenge for Russian air strikes against the jihadist group in Syria.

They argue the militants have neither the technology nor the expertise to down a plane flying at 30,000 feet (9,000 metres), although Germany's Lufthansa, Emirates and Air France have all halted flights over Sinai until the reasons for the crash were known.

Experts say human or technical error more likely caused the crash -- although they concede a surface-to-air missile could have struck the aircraft if it had been flying at lower altitude for some reason.

An Egyptian air traffic control official said the pilot told him in their last exchange that he had radio trouble, but Civil Aviation Minister Mohamed Hossam Kamal said communications had been "normal".