With concerns over security mounting, a number of European airlines readied to bring home thousands of tourists from the Sinai peninsula resort of Sharm el-Sheikh, where the crashed Russian plane took off from last Saturday.
The Islamic State (IS) jihadist group has claimed responsibility for the disaster, in which the Saint Petersburg-bound jet crashed minutes after taking off, killing all 224 mainly Russian tourists on board.
Cairo and Moscow have sought to downplay the suggestion of an attack.
But Obama told a US radio station: “I think there is a possibility that there was a bomb on board and we are taking that very seriously,” while emphasizing it was too early to say for sure.
In London, where Cameron hosted Egyptian President Abdel Fattah al-Sisi on Thursday, the British premier told reporters it was “more likely than not that it was a terrorist bomb” that caused the crash.
And The Times newspaper reported on Friday that electronic communications intercepted by British and US spies suggested a bomb may have been carried onto the plane.
A joint intelligence operation used satellites to uncover the chatter between militants in Sinai and Syria, it said.
“The tone and content of the messages convinced analysts that a bomb had been carried on board by a passenger or a member of the airport ground staff,” the newspaper reported, without giving a source.
The BBC’s security correspondent Frank Gardner said Britain’s security services suspect someone with access to the plane’s baggage compartment inserted an explosive device shortly before the plane departed.
But Egypt’s civil aviation minister Hossam Kamal said there was “as yet no evidence or data confirming the theory” of an attack and the Kremlin has dismissed the notion as “speculation”.
Russian President Vladimir Putin, who authorised strikes against fighters including IS militants in Syria, said assessments of the crash should be based on the “ongoing official investigation”, according to the Kremlin.
Sisi sought to use his trip to London to allay fears over the safety of tourists inEgypt after several nations suspended flights to and from Sharm el-Sheikh — leaving holidaymakers stranded while officials assessed security at the airport.
There is no global or European blanket ban and some flights have continued from the airport, but France and Belgium have warned citizens against travelling to Sharm el-Sheikh and Britain has advised against all but essential travel by air to or from the resort.
The British government has authorized flights to resume from the resort on Friday to bring home an estimated 20,000 British tourists — but passengers will only be allowed to carry hand luggage.
“The government has decided, in consultation with the airlines, that flights from Sharm to the UK will resume tomorrow,” a spokeswoman for Cameron said.
“The additional security measures will include permitting passengers to carry hand baggage only and transporting hold luggage separately.”
Belgian airline Jetair announced similar measures, while British airlines easyJet and Monarch said they would lay on extra flights to get customers home.
Joining a string of airlines in avoiding Sharm el-Sheikh, the Lufthansa Group announced its subsidiary Eurowings would halt flights between Germany and the Red Sea resort, while Turkish Airlines also cancelled two flights.
Flight KGL9268 was flying at altitude of 30,000 feet (9,150 metres) when it lost contact with authorities, 23 minutes after take-off from Sharm el-Sheikh to Russia’s second city.
Experts say the fact that debris and bodies were strewn over a wide area indicates the aircraft disintegrated in mid-air, meaning the crash was likely caused by either a technical fault or an explosion on board.
If it was behind the attack, it would be the first time IS, which controls large areas of Syria and Iraq, has attacked a passenger plane.
It has the potential to deeply damage Egypt’s tourism industry, still struggling to recover from a turbulent few years following the revolution of 2011.