Source: Russian plane black box points to bomb
The flight data and voice recorders showed "everything was normal" until both failed at 24 minutes after takeoff, pointing to "a very sudden explosive decompression."
The data "strongly favours" the theory of a bomb on board, the source said.
Meanwhile, the US Department of Homeland Security stepped up security procedures on US-bound flights from the Middle East on Friday, according to media reports.
US President Barack Obama and Britain's Prime Minister David Cameron both believe a bomb may have downed a Russian plane in Egypt, with reports Friday 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.
|The planed crashed into the Sinai desert on 31 October [Getty]|
The Islamic State [IS] extremist 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.
But Cairo and Moscow have sought to downplay the suggestion of an attack.
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 premier told reporters it was "more likely than not that it was a terrorist bomb" that caused the crash.
UK's 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.
Both Egypt and Russia have warned blaming a terrorist attack for the plane crash is "premature" and said these allegations could further devastate Egypt's tourism industry.
Vladimir Putin's spokesperson, Dmitry Peskov, said on Thursday that any theories about what caused a Russian plane to crash in the Sinai Peninsula "are speculation" and that "only the official investigation can determine what happened".
Russia's foreign ministry have also complained about Britain's failure to hand it information about the plane crash.
"If they have information and they are not presenting it that is shocking," Maria Zakharova, a foreign ministry spokesperson, told reporters.
Egypt's foreign minister, Sameh Shoukry, has said the UK's move to suspend flights from Sharm al-Sheikh was "premature and unwarranted", and risked devastating consequences for the country's vital tourism industry.
Egyptian President Abd al-Fattah al-Sisi met with British Prime Minister David Cameron at 10 Downing Street on Thursday, where they held a joint press conference.
Egypt's president and Britain's prime minister both agreed on the need for "the tightest possible security" at the Sinai resort airport where the doomed Russian airliner departed.
'Jihadi' online chat
|IS claimed responsibility for the crash [Twitter]|
A US counterterrorism official told CNN that the "specificity" in the online chatter surrounding the crash of the Russian jet and "the specific nature of the discussion" being monitored drew the attention of the US intelligence community.
The official also said the communications included people associated with the IS-affiliated Sinai-based militant group Wilayat Sinai, talking about the bomb's origin and bragging about the crash.
Wilayat Sinai supposedly released an audio tape on Tuesday that claimed that they downed the aircraft and challenged air crash investigators to prove them wrong.
Wilayat Sinai [Sinai Province], the IS franchise formerly known as Ansar Bayt al-Maqdis, has killed hundreds of police officers and soldiers in North Sinai since the Egyptian military overthrew Islamist President Mohammad Morsi in 2013.
The group, which aims to topple the government, has mainly focused on targets in Sinai but has claimed responsibility for attacks in Cairo, and in other provinces