The source declined to give more details but based his comments on the preliminary examination of the black boxes recovered from the Airbus A321 which crashed in the Sinai Peninsula on Saturday killing all 224 people on board.

The Egyptian government said the black boxes were being examined by Egyptian and Russian experts along with German and French specialists from Airbus and from Ireland where the aircraft was registered. It said the search was continuing across the 9-sq-km crash site. Security sources said intelligence agencies had obtained a copy of the passenger list.

Russian officials have said the plane, carrying holidaymakers from the Red Sea resort of Sharm el-Sheikh to St Petersburg, likely broke up in mid-air but said it was too early to say what caused it to crash.

The first bodies recovered from the wreckage arrived on board a Russian government plane at St Petersburg's Pulkovo Airport, where grieving Russians left piles of flowers.

A Reuters photographer saw a white lorry leaving the airport, escorted by police cars, heading for a St Petersburg morgue, where the bodies were to be identified. Egypt said the plane was carrying 196 bodies. A second plane was due to leave Cairo today evening.

Russian President Vladimir Putin, who had declared Sunday a day of mourning, said today the crash was a great tragedy.

"Without any doubt everything should be done so that an objective picture of what happened is created," Putin said in comments cited by ITAR-TASS. "So that we know what happened."

When asked if a terrorist attack could be to blame, Putin's spokesman Dmitry Peskov said no theory could yet be ruled out.

An Egyptian militant group affiliated with Islamic State said on Saturday it brought down the plane "in response to Russian air strikes that killed hundreds of Muslims on Syrian land". Russian Transport Minister Maxim Sokolov dismissed the claim, saying it "can't be considered accurate".

'No pilot error'

Alexander Smirnov, deputy general director of airline Kogalymavia, which operated the plane under the brand name Metrojet, said only a "technical or physical action" could have caused the aircraft to break up in the air.

"The plane was in excellent condition," Smirnov told a news conference in Moscow. "We rule out a technical fault and any mistake by the crew."

Kogalymavia's deputy general director for engineering, Andrei Averyanov, said a 2001 incident when the plane's tail section struck the tarmac on landing was fully repaired and could not have been a factor in the crash.

The airline said the plane's engines were inspected in Moscow on Oct. 26 and no problems were found and a Russian inspection of its fuel found that it met requirements.

The aircraft had received a certificate of airworthiness earlier this year from regulators in Ireland.

Russia, an ally of Syrian President Bashar al-Assad, launched air raids against opposition groups in Syria including Islamic State on September 30.

Islamic State, the hardline group that controls swathes of Iraq and Syria, has called for war against both Russia and the United States in response to their air strikes in Syria.

Sinai is the scene of an insurgency by militants close to Islamic State who have killed hundreds of Egyptian soldiers and police and have also attacked Western targets in recent months.

However, militants in the area are not believed to have missiles capable of hitting a plane at 30,000 feet.

Islamic State websites have in the past claimed responsibility for actions that have not been conclusively attributed to them.

James Clapper, US director of National Intelligence, said in Washington: "We don't have any direct evidence of any terrorist involvement, yet." On the ability of Islamic State militants to shoot down an airliner, Clapper said: "It's unlikely, but I wouldn't rule it out."

Those on board the flight included 214 Russians, at least three Ukrainians and one Belarusian, most returning from the Red Sea, popular with Russians seeking winter sun.

Russia and other former Soviet republics have poor air safety records, notably on domestic flights. Some accidents have been blamed on the use of ageing aircraft, but industry experts point to other problems, such as poor crew training and lax government controls.

The A321 is a medium-haul jet in service since 1994, with more than 1,100 in operation worldwide and a good safety record.

The aircraft disappeared from radar screens 23 minutes after take off at an altitude of 31,000 feet (9,400 metres), Egypt's Civil Aviation Ministry said. FlightRadar24, a Sweden-based flight tracking service, said the aircraft was descending rapidly when the signal to air traffic control was lost.