* Russia says deeply concerned over possible U.S. intervention
* Evidence has been destroyed, U.S. and its allies say
* Western countries say strong response needed
* U.N. says Syria has agreed to ceasefire during inspections (Adds Assad interview, Russian and French comments)
By Erika Solomon
BEIRUT, Aug 26 (Reuters) - U.N. inspectors left central Damascus on Monday to investigate sites of an alleged chemical weapons strike on the outskirts of the Syrian capital, a Reuters witness said, after calls from Western powers for military action to punish what may be the world's worst chemical attack in 25 years.
Syria agreed on Sunday to allow the inspectors to visit the site. But the United States and its allies say evidence has probably been destroyed by heavy government shelling of the area over the past five days. It said the offer to allow inspectors came too late.
The six-car convoy of chemical weapons experts wearing blue U.N. body armour was accompanied by a car of security forces as well as an ambulance. They said they were headed to the rebel-held outskirts known as Eastern Ghouta, where activists say rockets loaded with poison gas killed hundreds of people early on Wednesday.
President Bashar al-Assad, who has been fighting a 2-1/2-year revolt, said accusations that his forces used chemical weapons were politically motivated and warned the United States against intervening in his country.
"Would any state use chemicals or any other weapons of mass destruction in a place where its own forces are concentrated? That would go against elementary logic. So accusations of this kind are entirely political," he told the Russian newspaper Izvestia in an interview.
"Failure awaits the United States as in all previous wars it has unleashed, starting with Vietnam and up to the present day."
The United Nations said Damascus agreed to a ceasefire while the U.N. experts are at the site for inspections.
Activists in Ghouta said that rebels had also agreed to halt operations and several brigades would provide protection to the visiting U.N. team.
But as one activist spoke to Reuters by Skype, the sound of exploding mortar shells could be heard in the distance - highlighting the dangers and difficulties inspectors may face as they try to investigate.
"We've agreed to halt our actions and this morning has been much quieter, but we're still getting occasional mortar strikes and there has been one air raid," said activist Abu Nidal.
Syria's conflict has so far been met with international deadlock. The growing violence has killed more than 100,000 people, stoked regional sectarian violence, and revived Cold War-era divisions between Western powers and Russia and China
Washington has faced growing calls for action in response to Wednesday's attack, which came a year after President Barack Obama declared use of chemical weapons to be a "red line" which would require a firm response.
Russia, Assad's main arms supplier, says rebels may have been behind the chemical attack and said it would be a "tragic mistake" to jump to conclusions over who was responsible.
Its Foreign Ministry said on Monday that it was concerned about a potential U.S. military response and urged Washington to refrain from falling for "provocations".
Iran, the regional Shi'ite Muslim power that has been bankrolling Assad against a revolt led by Syria's Sunni Muslim majority, announced its own "red line", warning Washington of "severe consequences" if it intervened in Syria.
U.S. officials stressed that Obama has yet to make a decision on how to respond. A senior senator, Republican Bob Corker, said he believed Obama would ask Congress for authorisation to use force when lawmakers return from summer recess next month.
France said on Monday morning that there had been no decision yet on military action.
"There has to be a proportional reaction ... and that will be decided in the coming days, Foreign Minister Laurent Fabius told Europe 1 radio. "All options are envisaged. The only one that is not on the table is to not do anything."
Underlining diplomatic difficulties in forging international agreement, he noted that Russia and China would probably veto a U.N. Security Council move to strike Assad, creating a potential problem under international law for any assault.
British Foreign Secretary William Hague, however, said that it would be possible to respond to a chemical weapon attack without the Security Council's backing.
Obama has been reluctant to intervene in a conflict which began as protests against four decades of Assad family rule but grew into a civil war overtaken by sectarian bloodshed and a strengthening Islamist insurgency with links to al Qaeda.
The death toll of civilians caught in the midst of the violence rises by the hundreds daily. Activist estimates for the alleged poison gas attack ranged from 500 dead to well over 1,000, which would make it the worst chemical weapons attack since Saddam Hussein gassed and killed thousands of Iraqi Kurds at Halabja in 1988.
Turkey, a former Assad ally that is now a major backer of the opposition, said it would join any international coalition even if a decision for action could not be reached at the U.N.
The U.N. experts had arrived in Syria to investigate smaller suspected chemical strikes just three days before the Aug. 21 incident, which occurred before dawn after a night of heavy bombardment.
For days, the team was waiting in its Damascus hotel just a few miles away before the Syrian government agreed to allow it access to the sites.
Syria's information minister said the government had evidence chemical arms were used by rebels not Assad's forces. Western states say they believe the rebels lack access to poison gas or weapons that could deliver it.
The experts' mandate is to find out whether chemical weapons were used, not to assign blame, but the evidence they collect, for example about the missile used, can provide a strong indication about the identity of the party responsible.
If the U.N. team obtains independent evidence, it could be easier to build an international diplomatic case for intervention. Former weapons investigators say every hour matters. (Editing by Mohammad Zargham, Christopher Wilson and Alastair Macdonald)