British Prime Minister David Cameron said on Thursday it was increasingly likely a bomb brought down a Russian airliner over Egypt with the loss of 224 lives, and U.S. President Barack Obama said Washington was taking that possibility "very seriously".
But Moscow, which launched air strikes against Islamist fighters including Islamic State in Syria more than a month ago, said it was premature to reach conclusions that the flight was attacked.
In a telephone call, Russian President Vladimir Putin told Cameron it was important that assessments of the cause of the crash last Saturday be based on information from the official investigation, Interfax news agency reported.
Egypt, which depends on tourism as a crucial source of revenue, said there was no evidence a bomb was to blame.
A Sinai-based group affiliated with Islamic State, the militant group that has seized swathes of Iraq and Syria, has claimed responsibility for the crash, which if confirmed would make it the first attack on civil aviation by the world's most violent jihadist organization.
Cameron, who hosted Egyptian President Abdul Fattah al-Sisi on Thursday for a previously scheduled visit, said: "We cannot be certain that the Russian airliner was brought down by a terrorist bomb, but it looks increasingly likely that was the case."
His foreign secretary, Philip Hammond, said it was "a significant possibility" that Islamic State was responsible, given a range of information, including the claim of responsibility.
In his first public comments on the disaster, Obama said in a radio interview: "There's a possibility that there was a bomb on board. And we're taking that very seriously."
"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 KIRO/CBS News Radio in Seattle.
Britain, Ireland, Germany and the Netherlands have suspended flights to and from Sharm al-Sheikh, leaving thousands of European tourists stranded in the Red Sea resort where the plane took off. A spokesperson for Cameron said later that flights from the resort destination to Britain would resume on Friday.
Cairo said the decision to suspend flights was unjustified and should be reversed immediately.
Britain said it was working with airlines and Egyptian authorities to put in place additional security and screening measures at the airport to allow Britons to get home.
If a bomb brought down the Airbus A321 (AIR.PA), that would devastate Egypt's tourism industry, still recovering from years of political turmoil. Shares in holiday companies Thomas Cook (TCG.L) and TUI Group (TUIT.L) fell.
ABC News, citing government and aviation officials, said the U.S. Department of Homeland Security was expected to call for tighter security, possibly as soon as Friday, at certain foreign airports that have direct flights to the United States. DHS said it had no announcements to make for now.
While Egypt has bristled at the suspensions of flights, Sisi said during his visit to London that he understood concerns about safety. He said Cairo had been asked 10 months ago to check security at the airport in Sharm al-Sheikh.
"We understood their concern because they are really interested in the safety and security of their nationals," he added.
Dmitry Peskov, Putin's spokesman, said Russian planes were still flying to and from Sharm al‐Sheikh.
"Theories about what happened and the causes of the incident can only be pronounced by the investigation," Peskov said.
Egypt's civil aviation minister, Hossam Kamal, said investigators had no evidence so far to support the explosion theory. Russian aviation agency Rosaviatsia said investigators would examine whether there was any explosive material on the plane.
Security experts and investigators have said the plane is unlikely to have been struck from the outside and Sinai militants are not believed to have any missiles capable of striking a jet at 30,000 feet. Russia's Kogalymavia airline, which operated the plane, said three of its four remaining A321 jets had passed Russian safety checks, while the fourth would be checked shortly.
Russia on Thursday began burying some of those killed in the crash, which could affect strong public support for the Kremlin's air strikes in Syria.
In St Petersburg, the intended destination of the flight, friends and loved ones bade farewell to 31-year-old Alexei Alexeyev, who worked for a heating and ventilation company and had been returning from a holiday.
Islamic State has called for war against both Russia and the United States in response to their air strikes in Syria. Egyptian intelligence officials said the Islamic State branch suspected in the downing of the airplane had eluded a security dragnet by operating in secretive cells inspired by a leader who was once a clothing importer.
The Sinai Province group has focused on killing Egyptian soldiers and police since the military toppled President Mohamed Mursi of the Muslim Brotherhood in 2013 after mass protests.
A senior Russian lawmaker said Britain's decision to stop flights from Sharm was motivated by London's opposition to Russia's actions in Syria.
"There is geopolitical opposition to the actions of Russia in Syria," said Konstantin Kosachev, a senior member of Russia's upper house of parliament, when asked about Britain's decision, in comments reported by RIA news agency.
At Sharm airport, security appeared to have been tightened on Thursday with security forces patrolling the terminals and not allowing drivers, tour agents or others to loiter while awaiting tourist arrivals, a witness said.