CAIRO (AFP) - Britain said Wednesday it was concerned a Russian airliner that crashed in Egypt may have been downed by a bomb, as the Islamic State in Iraq and Syria (ISIS) group insisted it caused the disaster.

London's announcement came as Egyptian officials said investigators probing the black boxes had extracted the data from one for analysis, but the other had been damaged and required a lot of work.

"While the investigation is still ongoing we cannot say categorically why the Russian jet crashed," British Prime Minister David Cameron's office said.

"But as more information has come to light, we have become concerned that the plane may well have been brought down by an explosive device."

Britain temporarily suspended flights from Egypt's Red Sea resort of Sharm el-Sheikh.

It was from there that the Metrojet Airbus A-321 that crashed killing all 224 people on board took off on Saturday.

"We recognise that this information may cause concern for those in Sharm and indeed for those planning to travel to Sharm in the coming days," Downing Street said, adding that Cameron would hold an emergency Cabinet meeting on the issue later Wednesday.

In Dublin, the Irish Aviation Authority told the country's airlines not to fly to Sharm el-Sheikh or over the Sinai "until further notice".

Within hours of the crash, the militant ISIS group claimed it downed the plane but provided no details.

Both Russia and Egypt doubted the claim, but officials say they cannot rule out anything and that investigation of the black boxes may take weeks or even months.

In a new statement Wednesday, ISIS insisted it had brought down the plane - and challenged sceptics to prove otherwise.

"We are under no obligation to explain how it came down," the group said in an audio statement posted online.

"Prove that we didn't bring it down, and how it came down. We will detail how it came down at the time of our choosing."

Cameron's office said he and Egyptian President Abdel Fattah al-Sisi agreed in a phone call Tuesday on the need for tight security at Sharm el-Sheikh airport.

Cameron and Sisi "agreed it was important not to pre-judge the investigation", the British statement said.

They "noted that there was still uncertainty about the cause of the crash and agreed it would be prudent to ensure the tightest possible security arrangements at Sharm el-Sheikh airport as a precautionary measure."

The ISIS affiliate in Egypt is waging a bloody insurgency in the north of the Sinai Peninsula that has killed hundreds of policemen and soldiers.

On Wednesday, it claimed responsibility for a suicide car bombing on a police social club in El-Arish, which the official Mena news agency said killed four policemen.

Sisi, who left for London Wednesday, has downplayed as propaganda the ISIS claim of responsibility for the crash but said a full investigation will take time.

US director of national intelligence James Clapper also said this week it was "unlikely" ISIS was involved, but he did not exclude the possibility.

The airline itself has ruled out a technical fault or human error, drawing fire from the head of Russia's aviation authority.

Alexander Neradko said the airline's conclusions were "premature and not based on any real facts".

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

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

American 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 in-flight event, possibly a bomb explosion.

Search operations had been extended to a radius of about 40km.

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

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

In the city centre, about 300 people attended a vigil for the victims on Wednesday.

"We are here to honour those who died. This is not the time to say who is to blame, the most important thing is to express our common grief," said activist Valentin Podsvin.

ISIS has deployed shoulder-fired anti-aircraft missiles in the past, but they are not known to possess weapons that could bring down an airliner at high altitude.

Militants have staged many attacks since the military overthrew Islamist president Mohamed Morsi in 2013.

They say these are retaliation for an ensuing police crackdown in which hundreds of Morsi supporters have been killed and thousands, including the ousted president, jailed.