Russia will give the Security Council evidence implicating Syrian rebels in a chemical attack on 21 August, Foreign Minister Sergei Lavrov has said.
Syrian officials supplied the evidence, which Mr Lavrov has not yet seen.
A UN report released on Monday concluded the nerve agent sarin was used in the attack in Damascus, which the US blames on the Syrian regime.
Russia has called the report one-sided and biased. The UN has hit back, saying its findings are "indisputable".
The UN report did not apportion blame for the attack, which sparked diplomacy that culminated in a deal for Syria to hand over its chemical arsenal by mid-2014.
The UK, France and the US now want the disarmament deal enshrined in a UN resolution backed by the threat of military force.
But Russia, which has repeatedly cast doubt on the whether the regime carried out the attacks, has objected to any resolution authorising force.
Meanwhile, fighting is continuing across Syria:
- Rebel groups are fighting each other in a town near the Turkish border, with al-Qaeda linked jihadists gaining the upper hand in a battle with the Free Syrian Army
- Unconfirmed video footage shows parts of Damascus being hit in air strikes
- The UK-based Syrian Observatory for Human Rights said troops were battling rebels near the motorway leading to Damascus airport, and Kurdish gunmen had forced jihadists from a village in the north
Mr Lavrov said there was plenty of evidence that pointed to rebel involvement in chemical attacks, including the Damascus assault.
"We will discuss all this in the Security Council, together with the report which was submitted by UN experts and which confirms that chemical weapons were used. We will have to find out who did it," he said.
Russia is Syria's most important international ally, and has three times blocked resolutions criticising the regime over the civil war in which the UN says more than 100,000 people have died.
Earlier Mr Lavrov's deputy, Sergei Ryabkov, said he had been given the evidence during a trip to Syria.
He said it needed to be analysed, and gave no details of its content.
Mr Ryabkov criticised the UN report, saying it was "distorted" and "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," he said.
"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."
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 they were later ordered to shift their focus to the Damascus incident, which was the most deadly chemical assault.
They are due to return to Syria "within weeks" to complete their inquiry into the other attacks, and a report is due in October.
Chief UN weapons inspector Ake Sellstrom, who wrote the report, told the BBC he thought Russia was not criticising the report itself but the process.
He described Mr Ryabkov's criticism as a political matter, and therefore not his remit.
Ban Ki-moon's office said the report's authors had the "fullest confidence" of the secretary general.
"They have worked impartially and to the highest scientific standards despite the exceptionally difficult conditions of the war in Syria," said Mr Ban's spokesman Martin Nesirky.
French Foreign Minister Laurent Fabius said he was surprised by the Russian reaction, adding: "Nobody can question the objectivity of the people appointed by the UN."
Human Rights Watch has taken the trajectory of the rockets from the UN document and plotted their likely path.
The rights group said the likely launch site for the missiles was in a government military compound.