Evidence cited so far does not meet the UN’s standard of proof of use of chemical weapons in the Syrian conflict, according to experts.
The statement came as Syrian information minister Omran al-Zoubi accused the opposition and their western allies of lying about the deployment of such weapons.
The Hague-based Organisation for the Prohibition of Chemical Weapons (OPCW) said the use of chemical weapons could only be determined if inspectors were given access to sites to collect soil, blood, urine and tissue samples, maintained custody of the samples, and examined them in certified laboratories.
Formal assessment
"This is the only basis on which the OPCW would provide a formal assessment," said spokesman Michael Luhan.
Tiny samples presented by opposition activists to US and British intelligence agencies have allegedly shown traces of sarin, a highly volatile colourless, odourless nerve agent.
When UN experts were asked by Damascus to go to the village of Khan al-Asal to investigate the possible use last month of chemical weapons by rebels, the UN initially agreed, but then demanded the establishment of a permanent team that would be given access to the entire country.
Mr Zoubi said this would repeat the Iraq scenario ahead of the 2003 US war.
“We do not trust American and British experts from a political point of view. We also do not trust their qualifications. Their aim is to juggle with facts,” he said.
To ensure neutrality, Mr Zoubi insisted that Russian and Chinese experts be added to the UN team, which is waiting in Cyprus. The exclusion of Russian and Chinese investigators was criticised by Russian envoy Vitaly Churkin, who proposed instead the exclusion of experts from Nato countries.
The Syrian opposition National Coalition, which rejects Russian and Chinese participation, claimed government forces fired “missiles with chemical arms” near Damascus last Thursday and Friday, and called for an investigation.
Embargo expiry
The coalition has been ramping up chemical weapons charges ahead of EU consideration of the expiry at the end of May of the embargo on arms for rebels.
Accusations that chemical agents have been used should not be justification for military action, said Russian deputy foreign minister Mikhail Bogdanov.
He urged immediate presentation of evidence so it could be checked “in conformity with international criteria” and not be used to “achieve other objectives”.
While US president Barack Obama has said the introduction of chemical weapons would be a “red line” that the government should not cross, he has also called for caution.