The United States and Britain on Wednesday said a bomb may have brought down a Russian airliner which crashed in Egypt, as the Islamic State group insisted it caused the disaster.
"A bomb is a highly possible scenario," a US official told AFP, four days after the Russian Airbus crashed in the Sinai Peninsula, killing all 224 people on board.
"It would be something that ISIL would want to do," he added, using an alternate name for the Islamic State (IS) group.
If confirmed, it would be the first ever Islamic State bomb attack on a passenger plane.
British Prime Minister David Cameron's office said in an earlier statement that "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 suspended all flights to and from Egypt's Red Sea resort of Sharm el-Sheikh, from where the ill-fated plane took off on Saturday, and warned against all but essential air travel to the area.
The US and British claims 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.
Ireland's aviation authority on Wednesday instructed airline operators not to fly to Sharm el-Sheikh or over the Sinai peninsula following the British announcement.
Air France and Lufthansa had already decided to stop flying over Egypt's Sinai peninsula as a precautionary measure while investigations into continued.
- 'Prove we didn't do it' -
Both Russia and Egypt have raised doubts over the claim of responsibility by IS, 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, IS again insisted it had brought down the plane -- and challenged sceptics to prove otherwise.
"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," the group said in an audio statement posted online.
The IS affiliate in Egypt is waging a bloody insurgency in the north of the Sinai Peninsula that has killed hundreds of policemen and soldiers.
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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.
The British decision to suspend flights came during a visit to Britain by Egyptian President Abdel Fattah al-Sisi.
Cameron's office said he and the Egyptian President had 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.
Russian airline Kogalymavia, which operated the plane, has ruled out a technical fault or human error, drawing fire from the head of Russia's aviation authority for a "premature" assessment.
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.
Search operations had been extended to a radius of about 40 kilometres (25 miles).
British aviation experts have been dispatched to Sharm el-Sheikh to assess the security situation.
- Identifying bodies -
Russian 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.
IS 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.