CAIRO: Investigators on Tuesday began examining the two black boxes from the Russian airliner that crashed in Egypt's Sinai Peninsula, as President Abdel Fattah al-Sisi said the probe would take time.

The Egyptian branch of the Islamic State (IS) jihadist group, however, claimed earlier that it had downed the Airbus A321 that crashed on Saturday killing all 224 people on board.

Read: Russian airliner carrying 224 crashed in Sinai

The examination of the black boxes - one which recorded on-board conversations and the other flight data - started at around midday, an Egyptian civil aviation ministry official told AFP.

The probe could last several weeks or months if the recordings in the black boxes have been damaged, sources said. Russia's government commission overseeing the investigation was also due to meet on Tuesday.

The Saint Petersburg-bound plane operated by Russian airline Kogalymavia crashed 23 minutes after taking off from the Red Sea resort of Sharm el-Sheikh.

Most of the passengers were Russian tourists.

Kogalymavia said the plane crashed due to “external action,” and that there was no technical fault or human error. It insisted the aircraft was in an “excellent technical condition”.

IS claim termed 'propaganda'

Within hours of the crash, the Egyptian affiliate of IS based in Sinai claimed it had downed the jet in retaliation for Russian air strikes targeting fellow jihadists in Syria.

“When there is propaganda that it crashed because of ISIS, this is one way to damage the stability and security of Egypt and the image of Egypt,” Sisi told the BBC, using an alternative acronym for IS.

“The plane was at an altitude of 35,000 feet. Believe me, the situation in Sinai - especially in this limited area - is under our full control.”

Sisi warned the probe could take years as in the case of Pan-Am flight 103 from London to New York that was brought down by a bomb and crashed into the Scottish town of Lockerbie in 1988.

“It takes time to clarify the incidents. You had the Pan-American that crashed over Europe. It took years before you reached the truth about the real reasons why it crashed,” Sisi said.

On Monday, US Director of National Intelligence James Clapper said it was “unlikely” that IS was involved in the Kogalymavia plane crash but did not rule out the possibility.

Alexander Neradko, head of Russia's aviation authority, criticised the airline's comments ruling out technical fault or human error, saying they were “premature and not based on any real facts”.

Cairo, Moscow and Washington have downplayed the IS claim, although analysts have not ruled out that a bomb may have been planted on board.

Experts say the fact that debris and bodies were strewn over a wide area points to a mid-air disintegration of the aircraft.

Also read: Russia mourns victims of crashed plane

IS militants in Sinai are not thought to have missiles capable of shooting down a plane at the altitude at which the Airbus was flying.

That leaves two possibilities - a technical fault that caused the plane to disintegrate, or an explosion caused by a bomb smuggled on board, according to experts.

US officials told CNN and other US television networks that a military satellite had detected a heat flash at the time of the crash, which could point to a catastrophic event during flight, possibly a bomb explosion although analysts were considering a range of potential causes.

Among other possibilities cited by CNN were the explosion of a malfunctioning engine or a structural problem with the plane, or wreckage hitting the ground.

Search operations have been extended to a radius of 40 kilometres.

“Every centimetre of the crash site should be inspected” to look for buried corpses and parts of the plane, Russian Emergency Minister Vladimir Puchkov said in a video conference with the Russian teams at the site, adding that drones and satellites could also be used if necessary.

'Huge tragedy'

President Vladimir Putin has described the crash - Russia's worst air disaster - as a “huge tragedy”.

“Without any doubt everything must be done to create an objective picture of events so that we know what happened and can react accordingly,” he said.

Relatives of the victims have begun the process of identifying the bodies after two planes delivered the remains of many of them to Saint Petersburg.

Also read: Bodies of 9 Egypt plane crash victims identified

Family members had already been providing DNA samples at a crisis centre set up near Saint Petersburg's Pulkovo airport, now the site of an impromptu memorial where people have brought flowers and cuddly toys to commemorate the victims, many of them children.