President Vladimir Putin vowed to hunt down those responsible for blowing up a Russian airliner over Egypt and intensify air strikes against Islamists in Syria, after the Kremlin concluded a bomb had destroyed the plane last month, killing 224 people.

"We will find them anywhere on the planet and punish them," Putin said at a somber Kremlin meeting broadcast on Tuesday. The FSB security service swiftly announced a $50 million bounty on the bombers.

Until now, Russia had played down assertions from Western countries that the Oct. 31 crash was the work of terrorists, saying it was important to let the official investigation run its course.

But four days after Islamist gunmen and bombers killed at least 129 people in Paris, Alexander Bortnikov, the head of the FSB, told a late night meeting that traces of foreign-made explosive had been found on fragments of the downed plane and on passengers' personal belongings.

"According to an analysis by our specialists, a homemade bomb containing up to 1 kilogram of TNT detonated during the flight, causing the plane to break up in mid air, which explains why parts of the fuselage were spread over such a large distance," said Bortnikov at the meeting in footage released by the Kremlin.

"We can unequivocally say it was a terrorist act," he said.

Egyptian authorities have detained two employees of Sharm al-Sheikh airport for questioning in connection with the downing of the Russian jet, two security officials and an airport employee said on Tuesday.

"Seventeen people are being held, two of them are suspected of helping whoever planted the bomb on the plane at Sharm al-Sheikh airport," said one of the security officials who both declined to be named.

The Airbus A321, operated by Metrojet, had been returning Russian holiday makers from Sharm al-Sheikh in Egypt to St Petersburg when it broke up over the Sinai Peninsula, killing all on board. A group affiliated with Islamic State claimed responsibility, but until Tuesday Russia had said terrorism was just one possible scenario.