US President Barack Obama raised the possibility on Thursday that a bomb brought down a Russian plane that crashed over Egypt's Sinai Peninsula, undermining efforts by Cairo and Moscow to downplay the suggestion of an attack.
"I think there is a possibility that there was a bomb on board and we are taking that very seriously," Obama told a US radio station, while emphasising it was too early to say for sure.
"We're going to spend a lot of time just making sure our own investigators and own intelligence community find out what's going on before we make any definitive pronouncements. But it's certainly possible that there was a bomb on board," Obama told the Seattle-based station.
And CNN cited multiple US officials as saying that according to the latest intelligence, a bomb planted onboard most likely caused the plane crash.
One official told CNN an employee at Sharm el-Sheikh airport may have assisted in helping to plant the bomb.
"This airport has lax security. It is known for that," the official said. "But there is intelligence suggesting an assist from someone at the airport."
With concerns over security mounting, a number of European airlines readied to bring home thousands of tourists from the Red Sea resort of Sharm el-Sheikh, where the plane took off last Saturday.
The Islamic State (IS) 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.
In London, where David Cameron was hosting Egyptian President Abdel Fattah al-Sisi, the British prime minister told reporters it was "more likely than not that it was a terrorist bomb" that caused the crash.
The Times newspaper reported on Friday that electronic communications intercepted by British and American spies suggested a bomb may have been carried onto the plane.
"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 has dismissed the notion as "speculation".
Russian President Vladimir Putin, who authorised strikes against fighters including IS militants in Syria, told Cameron via telephone that assessments of the crash should be based on the "ongoing official investigation".
Sisi sought to use his trip to London on Thursday to allay fears over the safety of tourists in his home country 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.
Outbound flights from Britain to Sharm el-Sheikh remain suspended but the British government authorised 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.
Russia on Thursday began burying the first victims of the crash, with several hundred people gathering in Veliky Novgorod, south of Saint Petersburg, to mourn 60-year-old Nina Lushchenko.
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.
IS, in claiming responsibility for the crash, said on Wednesday it would reveal how it had done so at a time of its choosing.
If confirmed, 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 turbulent years following the revolution of 2011 and the military coup of 2013.