Russia pounded Islamic State targets in Syria after confirming that a bomb attack brought down its passenger jet over Egypt last month, killing all 224 people on board.
President Vladimir Putin pledged to hunt down and "punish" those behind the attack, without blaming any specific group, as he ordered an intensification of Moscow's campaign in Syria and vowed "vengeance".
Russia's security agency announced a $50-million (47 million-euro) reward for information leading to the capture of those behind the attack on the Airbus jet, which crashed in the Sinai peninsula shortly after leaving Sharm el-Sheikh resort on October 31.
Cairo said it was enhancing security in airports around the country over the possibility the plane was "targeted by a terrorist attack", although the Egyptian probe into the disaster has yet to reach a conclusion.
"It is not the first time that Russia confronts barbaric terrorist crimes," Russian President Vladimir Putin said in a meeting with his security chiefs.
"The murder of our people in Sinai is among the bloodiest crimes in terms of victims," he said in comments released Tuesday, vowing to hunt down those responsible.
"We will search for them anywhere they might hide. We will find them in any part of the world and punish them," he said.
Islamic State jihadists have said they attacked the plane -- the deadliest assault on a Russian target since the Beslan school massacre by Islamist rebels from the North Caucasus in 2004 -- just days ahead of a wave of killings in Paris also claimed by the group that left at least 129 dead.
Putin and French leader Francois Hollande agreed in a phone call to "ensure closer contact and coordination" in their operations in Syria, the Kremlin said, as the Russian strongman ordered his navy to work with French forces in the Mediterranean "as allies".
Both countries have been targeting IS fighters, although the US-led coalition France is part of has sparred with Russia over its backing of forces loyal to Syrian President Bashar Al-Assad.
- 'Homemade' bomb -
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Russia's security chief Alexander Bortnikov told Putin that traces of explosives of "foreign production" had been found on the plane wreckage and that the jet carrying tourists back from Egypt was brought down by a home-made bomb with a force equivalent to one kilo of TNT.
"We can say unequivocally that this was a terrorist attack," Federal Security Service (FSB) head Bortnikov said.
The FSB later said it would pay "$50 million for information helping to arrest the criminals" and Russia's foreign ministry announced it was asking its "overseas partners" to help track down the perpetrators.
Russia has halted all flights to Egypt over security concerns after initially refusing to endorse suspicions in Britain and the United States that the plane was blown up.
Putin did not expressly blame IS for the attack on the passenger jet, but pledged to ramp up air strikes in Syria "so that the criminals understand that vengeance is inevitable."
Moscow on Tuesday for the first time sent its powerful long-range bombers to strike IS targets in the provinces around the jihadist strongholds of Raqa and Deir Ezzor and to fire cruise missiles at Idlib and Aleppo regions.
Defence Minister Sergei Shoigu said 206 "terrorist" targets had been hit in the latest wave of strikes.
The Pentagon said Russia had told the US about its strikes for the first time since Moscow began its bombing campaign on September 30.
"There was advance warning, giving us the opportunity if we had had aircraft in the area that we could have made adjustments," said press secretary Peter Cook.
France has also ratcheted up its strikes against IS targets around Raqa since the bloody attacks in Paris and Hollande will meet next week with Obama and Putin, as part of broad diplomatic push to end the war in Syria.
Putin has called for a united coalition against IS in Syria and on Monday reiterated that the attacks in France showed it was "indispensable" to join forces against the jihadists.