Russia Tuesday staged a 'significant number' of air strikes on the Islamic State group stronghold of Raqa in Syria, a US defense official said.

Russia on Tuesday staged a "significant number" of air strikes on the Islamic State group stronghold of Raqa in Syria, a US defence official said on the condition of anonymity.

He added that the attacks may have included sea-launched cruise missiles and long-range bombers and that Russia gave the United States advanced warning of the attacks, the official said.

The announcement came after President Vladimir Putin vowed vengeance as Moscow confirmed that it was a bomb which brought down a Russian passenger jet over Egypt last month, killing all 224 people on board.

Putin pledged to ramp up Moscow's bombing campaign in Syria in response to the attack. But he but stopped short of blaming any one group.

European Union nations on Tuesday also unanimously backed a French request for support with military missions in the wake of the Paris attacks, after France invoked a previously unused part of the bloc's treaties to seek help.

France, which launched new airstrikes against Islamic State militants in Syria after Friday's bloodshed, used a little-known article in the EU's Lisbon Treaty which provides for solidarity in the event one of them is attacked.

France is the first nation to invoke article 42.7 in the treaty, which requires all states to offer aid and assistance “as they are able”.

Meanwhile, Putin has offered 33 million pounds (around Rs 330 crore) reward for the capture of the people responsible for the crash of the Airbus A321M on 31 October, reported NDTV.

The Islamic State group has claimed responsibility for downing the Russian plane in written statements, as well as video and audio messages posted on the Internet following the crash.

"According to our experts, a homemade explosive device equivalent to 1 kilogram of TNT went off onboard, which caused the plane to break up in the air, which explains why the fuselage was scattered over such a large territory. I can certainly say that this was a terrorist act," said, FSB head Alexander Bortnikov.

He also mentioned that the tests showed the explosives had been produced outside of Russia, but gave no further details.

All of the people on board, most of them Russian tourists, were killed when the Metrojet Airbus 321-200 crashed over the Sinai Peninsula on Oct. 31, about 23 minutes after taking off from the Red Sea resort city of Sharm el-Sheikh. The plane was headed to St. Petersburg, where most of the passengers were from.

In Cairo, there was no immediate comment on the news from the Egyptian government. State-owned television carried the newsbreak from Moscow, but had no official comment either.

Egypt had resisted British and US assertions that an explosive device was the likely cause of the Russian plane's crash. Later, government officials and the pro-government media in Egypt shifted their focus away from the cause of the crash to speculating on what they called a Western conspiracy against Egypt and the crushing impact of the crash on the country's vital tourism industry.

Putin vowed to hunt down those responsible for the attack. "There's no statute of limitations for this. We need to know all of their names," vowed Putin. "We're going to look for them everywhere wherever they are hiding. We will find them in any place on Earth and punish them," he added.

The Islamic State group said that the attack was a retaliation for Russia's air campaign against IS and other groups in Syria, where Moscow wants to preserve the rule of President Bashar Assad.

Putin said on Tuesday that Russia's air campaign in Syria "should not only be continued but should be intensified so that the criminals realize that retribution is inevitable." He instructed the Defence Ministry and General Staff to present their suggestions on how Russia's operation in Syria could be modified.

"In this work, including the search to find and punish the criminals, we are relying on all of our friends," Putin said. "We will act in accordance with the U.N. Charter's Article 51, which gives each country the right to self-defence. Everyone who tries to aid the criminals should understand that they will be responsible for giving them shelter."

Russia made the announcement the day after meetings with other world leaders in Turkey, where they vowed to work together to combat the Islamic State group.

IS has warned Putin that it would also target him "at home," but did not offer any details to back its claim. While releasing specifics would add credibility, the group may be withholding because its claim is false, because doing so would undermine plans for similar attacks in the future, or because the aura of mystery might deepen its mystique among die-hard followers.

IS has also claimed responsibility for Friday attacks in Paris that killed 129 people and wounded 350 others.

With inputs from agencies