By Andrew Osborn

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

Putin ordered the Russian navy in the eastern Mediterranean to coordinate its actions on the sea and in the air with the French navy, after the Kremlin used long-range bombers and cruise missiles in Syria and announced it would expand its strike force by 37 planes.

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

Until Tuesday, 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, said in televised comments that traces of foreign-made explosive had been found on fragments of the downed plane and on passengers' personal belongings.

"We can unequivocally say it was a terrorist act," Bortnikov said at a Kremlin meeting.

Egyptian authorities have detained two employees of Sharm al-Sheikh airport, where the downed plane originated, for questioning, 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 the Egyptian resort to St Petersburg when it broke up over the Sinai Peninsula, killing all on board. A group affiliated with Islamic State claimed responsibility.

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RETRIBUTION

Putin, wearing a dark suit, presided over a minute of silence in memory of the victims at the Kremlin meeting, before telling security and military chiefs the incident was one of the bloodiest crimes in modern Russian history.

"Our air force's military work in Syria must not simply be continued," he said. "It must be intensified in such a way that the criminals understand that retribution is inevitable."

On Tuesday evening, Putin visited the defense ministry's command center in Moscow, to hear reports from military chiefs about what they were doing to implement his orders.

As dozens of uniformed servicemen watched on, the defense minister and top military officials gave Putin their reports one-by-one, reporting that long-range bombers had loosed 34 cruise missiles and that Russia would bolster its strike force of around 50 planes and helicopters with a further 37 aircraft.

"You are defending Russia and its citizens," Putin told military chiefs. "I want to thank you for your service and wish you luck."

Russia began air strikes in Syria at the end of September. It has always said its main target is Islamic State, but most of its bombs in the past hit territory held by other groups opposed to its ally, President Bashar al-Assad.

A senior French government source said Russia had launched air strikes against the Islamic State stronghold of Raqqa in northern Syria on Tuesday, signaling Moscow was becoming more concerned about the threat posed by IS.

A French defense official said Russia's realization that its plane had been felled by a bomb was a wake-up call for Moscow.

"What’s changed is less that France has changed, but that Russia has," said the official. "Russia has acknowledged that the plane was an attack carried out by Daesh (Islamic State). Russia ... is now beginning to say to itself that Daesh is also its enemy and has to be hit."

Putin, in language reminiscent of how he talked about Chechen militants during a war when he came to power 15 years ago, ordered the secret services to hunt down those responsible.

"We must do this without any statute of limitations and we must find out all their names," he said, invoking Russia's right to self defense under the United Nations charter.

"Anyone who tries to help the criminals should know that the consequences for trying to shelter them will lie completely on their shoulders."

(Additional reporting by Maria Kiselyova, Vladimir Soldatkin, Polina Devitt, Jack Stubbs and Daria Korsunksya in Moscow and by John Irish and Marine Pennetier in Paris; Editing by Christian Lowe and Peter Graff)