The Islamic State group said Wednesday it had smuggled a bomb on board a Russian airliner that went down last month, after discovering a "way to compromise the security" at an Egyptian airport.
The IS claim, published in the group's magazine, came a day after Moscow announced that traces of explosives had been found in the plane wreckage and vowed to hunt down the perpetrators.
The attack on October 31 -- the deadliest against a Russian target since the Beslan school massacre in 2004 -- killed all 224 people on board the plane as it flew over Sinai en route to Saint Petersburg.
On Wednesday, Russian President Vladimir Putin and his Egyptian counterpart Abdel Fattah al-Sisi agreed "close cooperation" between their security services, the Kremlin said.
In its online magazine Dabiq, IS published what it said was a picture of the explosive, apparently contained in a soda can, and a purported picture it said its fighters had obtained of passports belonging to dead passengers.
The magazine said IS had initially planned to down a plane belonging to a country from the US-led coalition targeting militants in Iraq and Syria.
But the militants decided to instead target the Russian plane departing the Red Sea resort of Sharm el-Sheikh after Moscow began an air campaign in Syria in late September, according to the English-language publication.
"After having discovered a way to compromise the security at the Sharm el-Sheikh International Airport and resolving to bring down a plane belonging to a nation in the American-led Western coalition against the Islamic State, the target was changed to a Russian plane," it said.
"A bomb was smuggled onto the airplane."
Kremlin spokesman Dmitry Peskov said Russian authorities had seen the picture that appeared in the magazine in mass media and were investigating.
And in Washington, the State Department said it was still awaiting the result of the Egyptian inquiry but was operating under the assumption that the plan was downed by "terrorist activity."
"We have no reason to doubt ISIL's claim of responsibility," State Department spokesman John Kirby said, using an alternate acronym for IS.
Egypt says a team probing the disaster has yet to find the cause.
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-'Crime of opportunity'-
Moscow began air strikes in Syria in September against what it said were "terrorist" targets including IS jihadists.
Syria's opposition has accused Moscow of seeking to bolster President Bashar al-Assad's regime and targeting anti-regime rebels.
IS, based in parts of Iraq and Syria, commands the loyalty of militants in Egypt's Sinai Peninsula who have killed hundreds of soldiers and policemen.
Their attacks have focused mainly on Egyptian security forces since the army ousted Islamist president Mohamed Morsi in 2013 and unleashed a bloody crackdown on his followers.
An attack of such magnitude against tourists as the plane bombing would signal a shift in tactics.
An analyst said IS could have decided on a Russian plane because it was easier, with Russian tourists outnumbering other nationalities that visit Egypt's Red Sea resorts.
"They make it sound like a last-minute decision based on Russia's entry into the Syrian conflict," said Mokhtar Awad, an analyst with the Center for American Progress, a US think tank.
"It may have actually been easier or more manageable on a Russian plane, as opposed to perhaps more scrutiny on a British flight," he said.
Russia has pounded IS targets in Syria since Tuesday, but had previously been accused of focusing more on the group's opponents among rebels fighting Assad.
"Picking Russia -- aside from being a crime of opportunity -- it is also the most hated foreign power among Syrians aside from Iran," Awad said, referring to Syrians who oppose Assad.
"Attacking one of Assad's primary backers helps improve the group's domestic image" against accusations that it is not fighting the regime, he said.
If the group had indeed placed a bomb on board the plane, in the heavily secured resort, it would cast doubts on aviation security in Egypt and perhaps other countries.
Egypt on Tuesday said it was "enhancing" airport security, including security sweeps of airplanes and checks on flight crews.