* Syria any air strikes would need coordination
* U.S. hinted it could extend fight into Syria (Adds background, quotes, Russia)
By Mariam Karouny and Sylvia Westall
BEIRUT, Aug 25 Syria said on Monday it would cooperate in any international effort to fight Islamic State militants, after Washington signalled it was considering extending the battle against the group into Syrian territory.
Russia, the most prominent foreign backer of Syrian President Bashar al-Assad, also urged Western and Arab nations to overcome their distaste of the government in Damascus and engage with it to fight the hardline insurgents.
Syrian Foreign Minister Walid al-Moualem held open the possibility of working with a range of countries, including the United States, Britain and Saudi Arabia, all of which supported the uprising against Assad.
Moualem presented his country as a vital partner in a war against Islamic State, which has seized areas of Syria and Iraq and declared a "caliphate" in the territories it controls.
"Syria, geographically and operationally, is the centre of the international coalition to fight Islamic State," Moualem said in a televised news conference. "States must come to it if they are serious in combating terrorism," he added.
Asked about the prospect of U.S. air raids against Islamic State inside Syria, Moualem said any strikes would have to be coordinated with Damascus. "Anything outside this is considered aggression," he told reporters.
Asked if Syria was ready to work with the United States and Britain in fighting the group, he said: "They are welcome."
He also called for intelligence sharing with neighbouring states and suggested cooperation would be possible with Saudi Arabia, another major backer of the anti-Assad uprising that has shown increasing alarm about Islamic State.
The White House signalled on Friday it was considering taking the fight against Islamic State into Syria after days of air strikes against the group in Iraq and the beheading of a U.S. journalist.
But Washington has also supported a more than three-year-old insurgency in Syria, which has killed at least 191,000 people, and there has been no sign of any shift in U.S. policy towards Assad. Britain has also ruled out negotiating with him.
"He's part of the problem," Ben Rhodes, President Barack Obama's deputy national security adviser, said last week. Last year, Washington came close to bombing Syria after accusing Assad's forces of using chemical weapons.
Islamic State, an offshoot of al Qaeda which draws some of its strength from foreign fighters, has emerged as the strongest group in the insurgency. Its thousands of fighters control roughly a third of Syria, with territory in the north and east.
Until recently, direct engagements between the Syrian army and Islamic State were rare. Activists and Western officials accused the Syrian army of leaving the group to its own devices as it was crushing less radical opposition factions.
The government has mostly focused its military efforts on beating back insurgents in a strategic corridor of territory stretching north from Damascus -- areas far from the group's strongholds where it has less of a presence.
The pattern of conflict has changed in recent weeks, with hundreds of government loyalist fighters killed in engagements with the group as it has sought to expand further in Syria, strengthened by weaponry brought in from Iraq.
Assad has characterised his opponents as extremists from the start of the uprising in 2011, when his forces violently suppressed protesters inspired by the Arab Spring. Critics say the reaction of security forces helped radicalize them.
On Sunday, Islamic State fighters captured an air base in northeast Syria after days of fighting that cost more than 500 lives, according to a monitoring group.
"Islamic State has no borders and the faster we move against it, the more we diminish its danger," Moualem said.
Moualem also condemned the killing of U.S. journalist James Foley, who was beheaded by the Islamic State, apparently in Syria. The Pentagon has said the U.S. military failed in a secret rescue attempt of Foley and other U.S. hostages.
"If it is confirmed and this military operation did take place and failed, I say if there were prior coordination then the possibility of its failure would have been very low," Moualem said. (Writing by Tom Perry/Sylvia Westall; Editing by Andrew Heavens and Crispian Balmer)