With concerns over security mounting, European airlines readied to bring home thousands of tourists from the Sinai peninsula resort of Sharm el-Sheikh, from where the crashed Russian plane took off last Saturday.
But there were angry scenes at the airport as thousands of anxious Britons who had hoped to fly home were sent back to their hotels after Egypt blocked additional repatriation flights.
A first flight took off for London's Gatwick airport after a long delay.
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.
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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 emphasising it was too early to say for sure.
And The Times newspaper reported today 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.
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 said Britain had not shared the intelligence on which its bomb suspicions were based.
"We don't know what data our British colleagues are basing themselves on," spokesman Dmitry Peskov told reporters.