The United Nations chief has said his organisation's report into the use of chemical weapons in Syria is "indisputable".
Russia has said the assessment was "biased" after receiving evidence from President Assad's government claiming rebel involvement in last month's attack.
However, Ban Ki-moon insists the document handed in by weapons inspectors is " thoroughly objective."
A Russian diplomat visiting Damascus said president Bashar Assad’s regime turned over material on the attack last month.
The August 21 attack precipitated the high tension over Syria’s chemical weapons and sparked a plan under which Syria is to abandon the weapons. UN. investigators confirmed chemical weapons were used but did not say by which side in the civil war.
Russian deputy foreign minister Sergei Ryabkov said that Syria told Russian officials the material “bears witness to the rebels participating in the chemical attack” but that Russia is not yet drawing any conclusions.
The UN report did provide trajectory data that suggested the chemical-loaded rockets that hit two Damascus suburbs were fired from the north west suggesting they came from nearby mountains where the Syrian military is known to have bases.
Human Rights Watch also said that the presumed flight path of the rockets led back to a Republican Guard base in the same area.
“Connecting the dots provided by these numbers allows us to see for ourselves where the rockets were likely launched from and who was responsible,” Josh Lyons, a satellite imagery analyst for Human Rights Watch. But, he added, the evidence was “not conclusive.”
Mr Ryabkov said Syria told him the material it handed over shows “rebels participating in the chemical attack”. He added that Russia has submitted to the UN Security Council abundant and credible evidence that suggests it was not the government that fired the chemical weapons.
“We are unhappy about this report, we think that the report was distorted, it was one-sided, the basis of information upon which it was built is insufficient,” he said, referring to the UN report.
Russia has been Syria’s main ally since the start of the conflict in March 2011, blocking proposed UN resolutions that would impose sanctions on Assad’s regime and opposing an attempt to authorise the use of force if Syria does not abide by the agreement to get rid of its chemical weapons.
Assigning responsibility for last month’s attack has become a heated international diplomatic issue. Russian Foreign Minister Sergey Lavrov and his French counterpart Laurent Fabius sharply differed on the topic after meeting in Moscow on Tuesday. Mr Lavrov said Moscow has reason to believe the attack was a provocation staged by the rebels, while Mr Fabius said the evidence clearly implicates the government side.
Meanwhile the chief UN chemical weapons inspector said his team will return to Syria “within weeks” to complete the investigation it had started before the gas attack.
In London, Nato Secretary-General Anders Fogh Rasmussen welcomed the recent American-Russian agreement on eliminating chemical weapons in Syria, saying he thinks the “credible threat” of military action was the real reason “why diplomacy got a chance.”
He said that in order to ensure implementation of that agreement, it is crucial for the United Nations Security Council to “expeditiously adopt” a firm resolution to act as a framework for the process.