* Chief U.N. inspector says pressed Damascus on its version
* Say hard to see how rebels could have weaponised toxins
BEIRUT, Jan 30 (Reuters) - Syrian authorities who blamed the opposition for a deadly sarin gas attack last August have failed to present a plausible theory for how the rebels could have obtained the nerve agent, a U.N. investigator said in an interview published on Thursday.
France, Britain and the United States accused the government of President Bashar al-Assad of carrying out the Aug. 21 attack in which hundreds of people, including women and children, were killed. Syria and its ally Russia said it was rebel fighters who unleashed the chemical weapon.
Without categorically saying which side was to blame, chief United Nations investigator Ake Sellstrom said it was "difficult to see" how the opposition could have weaponised the toxins.
He told CBRNe World, a specialist publication on chemical, biological, radiological and nuclear threats, that he had asked Syrian authorities several times to back up their claim that rebels fired the weapons.
"They have quite poor theories: they talk about smuggling through Turkey, labs in Iraq and I asked them, pointedly, what about your own stores, have your own stores been stripped of anything, have you dropped a bomb that has been claimed, bombs that can be recovered by the opposition? They denied that.
"To me it is strange. If they really want to blame the opposition they should have a good story as to how they got hold of the munitions, and they didn't take the chance to deliver that story," Sellstrom said, in rare public comments from a leading figure in the inquiry.
(Read the full report here: http://www.cbrneworld.com/_uploads/download_magazines/Sellstrom_Feb_2014_v2.pdf)
PRESSURE ON SYRIA
A report by the U.N. last year said that chemical weapons were probably used in five attacks its experts had investigated in Syria, where more than 130,000 people have been killed in nearly three years of civil war. It said sarin was probably used in four incidents, of which the biggest was the Aug. 21 attack in rebel-held suburbs of Damascus.
The inquiry only looked at whether chemical weapons were used, not who used them.
The attack led to a deal between Russia and the United States that resulted in Syria promising to give up its entire stockpile of chemical weapons by mid-2014.
But sources familiar with the matter told Reuters on Wednesday that Damascus had given up less than 5 percent of its chemical weapons arsenal and would miss next week's deadline to send all toxic agents abroad for destruction. [ID:nL2N0L32OI}
The United States and several Western governments are stepping up pressure on Syria to speed up the stalled handover.
"It seems that the movement of its chemical weapons has slowed down," French Foreign Minister Laurent Fabius told reporters. "The international community must be extremely vigilant that Syria keeps to its commitments."
Foreign governments will deliver a stern message on Thursday at talks held by the executive council of the Organisation for the Prohibition of Chemical Weapons, the Nobel Peace Prize-winning body in The Hague, sources at the meetings said. (Reporting by Oliver Holmes in Beirut, Anthony Deutsch in The Hague and John Irish in Paris; Editing by Mark Trevelyan)