Russia does not have conclusive evidence that chemical weapons have been used in Syria, Russian Security Council head Nikolai Patrushev said.
"In my view, no one has reliable information about the use of chemical weapons in Syria," he said after a meeting with Belarusian President Alexander Lukashenko.
"If anyone does, we would like them to show their evidence that such weapons have indeed been used."
He stressed there is no evidence that either party in the Syrian conflict has used chemical weapons - either the government or the opposition.
"However, it is crucial to address these questions and conduct an investigation," Patrushev said.
NATO Secretary General Anders Fogh Rasmussen addresses the media during his monthly media conference at the Residence Palace building in Brussels on Monday, May 6, 2013. - AP
NATO Secretary General Anders Fogh Rasmussen has said the organisation has no information as to who could have used chemical weapons in Syria. He expressed regret over the fact that Syrian authorities have not allowed UN inspectors into the country to look into the possible use of such weapons.
At the weekend, an interview was broadcast on a Swiss-Italian TV network in which a leading UN official on Syria said that the UN commission to investigate alleged human rights abuses in Syria has heard testimony that the rebels have used chemical weapons.
"According to the testimonies that we have collected, the rebels have used chemical weapons, by using sarin gas," Carla Del Ponte, commissioner of the UN commission to monitor and investigate reported human rights abuses in Syria, told Radiotelevisione svizzera.
Sarin is a highly toxic nerve agent, which was reportedly first used for military purposes during the Iran-Iraq war in the 1980s. The best-known sarin gas attack was carried out by the members of Aum Shinrikyo group on the Tokyo subway, killing 13 and injuring dozens.
The Syrian government and rebel forces have traded accusations of using chemical weapons in recent weeks.
Last month, the White House said in a letter to lawmakers that intelligence gathered by the US, with the help of opposition forces in Syria, shows the Syrian regime has used chemical weapons on a small scale, but this evidence was later questioned as unclear.
According to recent UN figures, since March 2011 when violent clashes broke out in Syria, about 70,000 people have died there, over 3.6 million people within Syria are classed as "internally displaced", and over 1.3 million have fled the country.