Syria has given Russia new “material evidence” that Syrian rebels used chemical weapons in an attack on 21 August, a Russian minister has said.
Deputy Foreign Minister Sergei Ryabkov also said a report by UN inspectors on the alleged use of chemical weapons was politicised, biased and one-sided.
He said the inspectors had only investigated the attack in Ghouta on 21 August, not three previous incidents.
The UN team found the nerve agent Sarin was used in the Ghouta attack.
The report did not apportion blame for the attack but Western nations blame the government forces of Syrian President Bashar al-Assad.
Damascus – backed by Russia – says opposition forces are to blame.
Meanwhile the chief UN weapons inspector, Ake Sellstrom, has told the BBC it will be difficult to find and destroy all of Syria’s chemical weapons, but he believes it is “doable”.
“Of course, it will be stressful work,” he added.
Mr Sellstrom said much depended on whether the Syrian government and the opposition were willing to negotiate.
In an interview with Russian media, Mr Ryabkov said the Assad government had given him new evidence that rebel forces had used chemical weapons.
He did not give any details of what those weapons were.
“Just now we were given evidence. We need to analyse it,” he told RT news organisation.
Mr Ryabkov also criticised the UN report, saying it was “distorted, it was one-sided, the basis of information upon which it is built is not sufficient, and in any case we would need to learn and know more on what happened beyond and above that incident of 21 August”.
“We are disappointed, to put it mildly, about the approach taken by the UN secretariat and the UN inspectors, who prepared the report selectively and incompletely,” he told the RIA news agency.
The UN inspectors were originally mandated to go to Syria to investigate three alleged chemical weapons attacks – at Khan al-Assal, Sheikh Maqsoud and Saraqeb.
But after the 21 August attack in Ghouta, on the outskirts of Damascus, their instructions changed – and the report they produced is based purely on that incident.
A further UN report on the other locations will not be released until October.
The UN experts were not required to apportion blame in their report. ButHuman Rights Watch says the document reveals details of the attack that strongly suggest government forces were behind the attack.
Human Rights Watch used the details about the direction some of the rockets are thought to have come from, and worked out their trajectory. Their results indicated that the rockets were likely to have come from an area near a well-established military base.
On Tuesday the five permanent UN Security Council members – France, the UK, the US, Russia and China – met in New York to discuss a resolution on Syria’s chemical weapons.
They were discussing a draft resolution put forward by the UK, France and the US.
Such a document is seen as a key step in a US-Russia brokered plan under which Syria will disclose its arsenal within a week and eliminate it by mid-2014.
However, there have already been key disagreements over the wording.
France, the UK and US want a resolution containing the threat of military action but Russia opposes this.
Russian Foreign Minister Sergei Lavrov said on Tuesday that any UN resolution on the Syrian chemicals issue should not contain the threat of military action.
A resolution under Chapter VII of the UN charter permits military action if other measures do not succeed. Chapter VI requires a purely negotiated solution.
The BBC’s Daniel Sandford in Moscow says Russia has delivered a promise from Syria to give up its chemical weapons, and it seems that at this stage Moscow does not feel like giving the Western allies anything more.
Russia and China have three times blocked Western-backed Security Council resolutions against Mr Assad.
More than 100,000 people have died since the uprising against President Assad began in 2011.
Millions of Syrians have fled the country, mostly to neighbouring nations. Millions more have been internally displaced.