Russia struck the Islamic State stronghold of Raqa in Syria on Tuesday with a "significant number" of strikes that included long-range bombers and sea-launched cruise missiles, US defense officials said.

"We are aware that over the past several hours Russia conducted a significant number of strikes in Raqa, some of which may have included sea-launched cruise missiles and long-range bombers," said one official, who made the remarks on condition of anonymity.

Russia's Defense Minister Sergei Shoigu said that powerful long-range bombers had been used to strike targets around the jihadist strongholds of Raqa and Deir Ezzor, and to fire cruise missiles at Idlib and Aleppo regions.

A second US official, again speaking on condition of anonymity, later told AFP that Russia had sent one "strike package" of long-range bombers and had also fired "about 20" cruise missiles from the Caspian Sea, all of which appeared to have landed in Syria.

He said the Russian bombers flew to Syria across Iraqi air space.

Russian President Vladimir Putin earlier vowed vengeance as Moscow confirmed a bomb brought down a Russian passenger jet over Egypt last month, killing all 224 people aboard.

When Russia fired cruise missiles at Syria last month, the Pentagon said four of them had crashed in Iran. Moscow denied the claim.

The United States and Russia have been waging separate air campaigns in civil war-torn Syria, and US officials said Moscow gave it prior warning before carrying out Tuesday's strikes.

In the days after Russia's bombing campaign began, Pentagon and Moscow officials drew up flight safety guidelines specifying how pilots should act if they cross paths.

In a meeting with his security chiefs, Putin pledged to ramp up Moscow's bombing campaign in Syria in response to the attack on the passenger plane. But he but stopped short of blaming any one group.

"We will search for them anywhere they might hide. We will find them in any part of the world and punish them," he said.

Key developments in the war against IS
Paris (AFP) Nov 17, 2015 - The main developments in the war against the jihadist Islamic State group in Syria and Iraq since August 2014:

International coalition

On August 8, 2014, US jets strike IS positions in northern Iraq in response to an appeal from Baghdad, in the first American military operation in the country since troops withdrew in late 2011.

In September, US President Barack Obama vows to build "a broad, international coalition" to defeat IS. On September 23, the US and its Arab allies launch air strikes against IS in Syria.

IS driven out of Kobane

On January 26, 2015, IS is driven out of the Syrian border town of Kobane after more than four months of fighting led by Kurdish forces backed by coalition air strikes.

Tikrit retaken, Palmyra seized

On March 31, Baghdad announces the "liberation" of Tikrit, 160 kilometres (100 miles) north of Baghdad.

But in May, IS takes Iraqi provincial capital Ramadi, and then Syria's famed ancient city of Palmyra, a UNESCO World Heritage site.

On June 16, Kurdish militia backed by Syrian rebel forces and coalition air strikes seize the key Syrian border town with Turkey of Tal Abyad from IS, which had occupied it for more than a year. Tal Abyad was one of two main transit points on a key supply route to de facto IS capital Raqa.

Turkey begins air raids

On July 24, Turkish warplanes bomb IS positions inside Syria and Kurdish militants in a dramatic tightening of Ankara's stance.

The Turkish raids are largely aimed at positions of the Kurdistan Workers Party in Iraq and Syria.

Turkey gives Washington the green light for US warplanes to fly operations over Syria from its key southeastern air base of Incirlik.

French, Russian strikes in Syria

On September 27, France carries out its first air strikes against IS in Syria, after being part of the US-led coalition bombarding IS in Iraq since September 2014.

On September 30, Russia launches air strikes in Syria against "terrorists" seen as helping its ally the Damascus regime. Washington accuses Moscow of mainly targeting Syrian rebels.

On November 10 Syria's army breaks a more than year-long jihadist siege of a military air base in the north, its first major breakthrough since Russia's air campaign began.

Iraq's Sinjar retaken

On November 13, Iraqi Kurds announce the "liberation" of Sinjar from IS in an assault backed by US-led strikes that cut a key jihadist supply line with Syria.

Washington, which had announced on October 30 it was sending around 50 military advisers to Syria, says it has probably killed IS executioner "Jihadi John".

On November 14 Washington for the first time targets IS in Libya, killing its local chief, an Iraqi known as Abu Nabil.

Aftermath of Paris attacks

On November 15, two days after the devastating terrorist attacks claimed by IS in Paris that killed 129 people, France bombs the IS stronghold of Raqa and intensifies its air strikes in Syria. Washington says it will intensify its efforts.

On November 17, Putin orders the Russian navy in the Mediterranean to work "as allies" with its French counterparts after President Francois Hollande said the French aircraft carrier Charles de Gaulle would be deployed to the eastern Mediterranean.

Moscow also says Putin and Hollande have agreed to step up cooperation between their military and intelligence services in Syria after the attacks in Paris and the October 31 plane crash in Egypt that Russia says was caused by a bomb.