US President Barack Obama and Britain's prime minister both believe a bomb may have downed a Russian plane in Egypt, with reports suggesting their theory was based on intercepted communications.
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.
In a sign of mounting fears about the security of baggage handling in Egypt, Dutch carrier KLM announced that it had banned check-in luggage on an early flight from Cairo, mirroring moves taken by several European airlines on Sharm flights.
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 emphasising it was too early to say for sure.
In London, where Prime Minister David Cameron hosted Egyptian President Abdel Fattah al-Sisi on Thursday, the British PM 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 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.
Sisi sought to use his trip to London to allay fears over the safety of tourists in Egypt 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, including all Russian ones, 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.