The Russian Health Minister Yuri Shevchenko has hinted at the origin of the mystery gas used in theMoscow theatre siege last Saturday.
"To neutralise the terrorists a substance based on Fentanyl derivatives was used," he was quoted by Interfax as telling a news briefing.
"I officially declare: chemical substances which might have fallen under the jurisdiction of the international convention on banning chemical weapons were not used during the special operation."
Russia has been under increasing international pressure since the siege ended on Saturday to reveal what gas had been used, mainly to help doctors treat the injured.
As of Wednesday 119 people taken hostage by Chechen separatists in the theatre had died, all but two of them from the effects of the gas.
U.S. ambassador Alexander Vershbow said on Tuesday he believed the gas was Fentanyl, a potent opium-based narcotic used as an anaesthetic.
Opiates affect pain receptors and cause sleepiness and, in high doses, can lead to respiratory failure.