It is being reported the sound of an explosion can be heard on the cockpit voice recorder recovered from the Russian plane that crashed on Egypt's Sinai Peninsula.
French television channel France 2 is quoting an investigator who had access to the recorder.
The channel reported the investigator as saying the sound was not consistent with an engine failure.
The Metrojet Airbus A321 was flying from Sharm el-Sheikh to St Petersburg when it came down in Sinai on Saturday last week. Most of the victims were Russian.
Militants from the Sinai Province group, linked to Islamic State, said they downed the plane, but not how. IS has called for a war against both Russia and the US over their air strikes in Syria.
Russia has joined the UK, Belgium, Germany and the Netherlands in restricting travel to Sharm el-Sheikh after intercepted "chatter" suggesting a bomb was put in the plane's hold prior to take-off, killing all 224 people on board.
It was understood that UK officials received intelligence based on intercepted communications between militants in the Sinai Peninsula, indicating an explosive device may have been put inside or on top of the luggage just before the plane took off.
An unnamed investigator told French news agency AFP that the plane's flight data recorder had shown that "everything was normal during the flight, absolutely normal, and suddenly there was nothing", suggesting a "violent, sudden" explosion.
Experts in Moscow were investigating pieces of debris from the crash site, Russian officials said.
Flight restrictions cause chaos
Russian president Vladimir Putin has asked for up to 50,000 Russian holiday-makers currently in Egypt to be brought home.
The Kremlin says Russia will work with Cairo to improve aviation security up to what it calls a proper level at which point flights could resume.
But it said this does not mean that Russia now viewed a terror attack as the most likely cause.
Egypt is one of the most popular holiday destinations for Russians and any decision to suspend flights would cause major logistical problems for Russia's airlines and tourists.
The Russian Travel Industry Union estimated there were around 50,000 Russian tourists currently in Egypt and said refunding cancelled tickets to Egypt could bankrupt Russian tour operators, the Interfax news agency reported.
Tourist agency Tez Tour, which estimated it sells about 15 percent of trips to Egypt from Russia, said 10,000 of its Russian clients were in Egypt.
"How are they (the authorities) going to bring people back? If people are at a resort and they come to them to say a plane was sent to take you back, they would say: 'no, we want to be on holiday for two more weeks, we're not going anywhere'. An evacuation order would be needed," said Tez Tour general director Vladimir Kaganer.
British attempts to bring home thousands of stranded tourists were thrown into chaos on Friday (local time) when Egypt reduced the number of flights it would allow to take them home.
The UK estimated that 19,000 of its nationals remained stranded in Sharm el-Sheikh. They were starting to be repatriated home, after British Prime Minister David Cameron halted flights to and from Sharm al-Sheikh.
Egypt's Minister of Civil Aviation Hossam Kamal said the operation to bring large numbers of British holidaymakers from their hotels to the airport and then put them on flights without their luggage was "a huge burden on the airport because its capacity does not allow for that".
Britain said there was a "credible threat" the plane had been brought down by a bomb, but has refused to comment further, citing long-standing rules about disclosing operational details about live investigations.
The fate of Egypt's tourist industry, a vital source of hard currency for a struggling economy, is at stake as well as the credibility of President Abdel Fattah al-Sisi's claims to have brought under control the militants fighting to topple his government.
The crash has put Egypt's airport security measures in the spotlight.
KLM introduced new security measures on its trips from Cairo to Amsterdam. Passengers will only be allowed to take hand luggage onto the flight, Egyptian airport security sources said on Friday (local time).
Several passengers instead opted to take different flights. KLM Flight 554 left Cairo on Friday morning (local time) with only 115 passengers out of its 247 registered ones as a result.
Tourism contributed more than 12 percent to Egypt's economy in 2013 and the latest measures will hit it hard, analysts said. One in five foreign tourists in Egypt is Russian.
US increases security
The US has announced it is stepping up security screening of items on US-bound flights from some airports in the Middle East. The Homeland Security statement did not name which airports it was targeting.
US officials said they planned to boost security on commercial flights to the United States from certain airports in the region but they would not be specific about which airports would be affected.
The new measures would include expanded screening of items put on planes, airport assessments and offers of security assistance. They said the steps were being taken out of an abundance of caution.
President Obama has said that the US was taking the possibility that a bomb caused the crash very seriously.