* Road cut to chemical stockpile, not clear if other access
* Rebels already seized abandoned chemical base nearby
* Chemical in precursor form, unlikely rebels could mix them
By Dominic Evans
BEIRUT, May 2 (Reuters) - A Syrian rebel offensive aimed at easing a government siege east of Damascus has brought fighting closer to the last declared stockpile of President Bashar al-Assad's chemical weapons, according to diplomats and activists.
Syria has been removing 1,300 tonnes of chemical weapons under a deal reached last year which averted Western military strikes, after a sarin gas attack on rebel-held suburbs around the Syrian capital in August.
But it has missed several deadlines to ship out the toxins - the last of which was April 27 - and has told the international mission overseeing the operation that one remaining chemical site remains difficult to reach because of the fighting.
Assad's Western foes suspect him of deliberately dragging out the process, but the rebel advance east of Damascus suggests there are genuine obstacles to getting the chemicals out.
"It's a very contested area," said one diplomat following the chemical elimination process, which is supervised by a joint mission of the United Nations and the Organisation for the Prohibition of Chemical Weapons.
Activists say rebels have clashed with Assad's forces between Dumair air base, which they said came under heavy rocket fire from the rebels, and Sayqal air base about 40 km (25 miles) further east where the chemicals are believed to be held.
While the rebel attack appears more focused on Dumair and on breaking the military stranglehold which Assad's forces have imposed closer to the capital, the fighting has increasingly isolated Sayqal and encroached towards it.
Activists said the fighters are from the Islamic Front - one of the largest and most powerful Islamist rebel coalitions - the Rahman Corps and Ahmad Abdo Brigades. The offensive appeared to be funded by Gulf Arab supporters, they said.
The diplomat said rebels have overrun the abandoned and emptied chemical base at Khan Abu Shammal, which lies between Dumair and Sayqal, and cut the road linking them.
He described the proximity of the fighting to Sayqal as worrying and said it was not clear whether there were alternative routes to evacuate the chemicals other than the road which was blocked by the rebels.
Sigrid Kaag, head of the joint UN-OPCW mission, said on Sunday that 92 percent of Syria's chemical stockpile had been destroyed or loaded onto ships at the Mediterranean port of Latakia to be eliminated elsewhere.
The remaining chemicals at Sayqal have yet to be packed into containers for the road journey to Latakia, the diplomat said.
It was unlikely they could be used by the rebels if they were captured, he added, because they were in unmixed precursor form. "It's highly doubtful the rebels would have the capacity to do that (combine them)," he said. "The main risk might be that they might move it out or sell it on."
Another Western diplomat said rebels understood they could face consequences if they changed the focus of their attack and tried to take the chemical base. He did not elaborate.
OPCW spokesman Christian Chartier confirmed that all remaining chemicals were still located at one site, which he did not identify.
"Of course the fighting is concerning, because it has prevented Syria and it looks like continuing preventing Syria to meet its obligations as quickly as possible," he said.
"This is not something we can control. We can only wish that Syria does whatever it can to move the chemicals so that we can start on the actual destruction process."
While Syria is tantalisingly close to shipping out all its declared chemical arsenal - a major feat amid a conflict which has killed 150,000 people - it has yet to agree with the OPCW on a plan to destroy 12 remaining chemical facilities.
The OPCW is also deploying a fact-finding mission to Syria to investigate accusations by rebels and activists of a series of chlorine gas attack in recent weeks.
Chlorine, a far weaker agent than sarin or mustard gas, was not included in Syria's declaration to the OPCW last year. But it is still deadly and its use in warfare is banned by the Chemical Weapons Convention which Syria signed up to last year.
Assad's Western and Arab opponents say forces loyal to the Syrian president, who has been battling rebels for three years, were responsible for the August 2013 sarin gas attacks which killed hundreds of people.
Syrian authorities reject the accusation, saying rebel fighters trying to trigger foreign military intervention carried out the world's worst chemical attack in a quarter century. (Additional reporting by Mariam Karouny in Beirut and Thomas Escritt in Amsterdam; Editing by Giles Elgood)