The Russian military says it had found no traces of a chemical attack in the Syrian town of Douma, Interfax news agency reports.
The defence ministry said on Monday Russian military medics had examined patients in a hospital in Douma and found no symptoms of chemical poisoning, according to the agency.
The UN human rights chief lambasted Security Council members on Monday for offering only "feeble condemnations" of a chemical attack in Syria, saying the failure to react more strongly could have dire consequences for decades.
"A number of very powerful states are directly involved in the conflict in Syria, and yet they have completely failed to halt this ominous regression towards a chemical weapons free-for-all," Zeid Ra'ad al-Hussein said in a statement.
"This collective shrug to yet another possible use of one of the most ghastly weapons ever devised by man is incredibly dangerous."
Witnesses and medical workers reported as many as 60 deaths in the rebel-held town of Douma on Saturday, with nearly a thousand injured, after at least two bombs hit a hospital and nearby buildings.
"Verbal condemnation is clearly grossly insufficient, and the abject failure to even properly investigate each and every allegation of chemical attacks by all sides further encourages the use of such despicable weapons and undermines the legitimacy of the international legal order," Zeid said.
There have been at least 35 chemical weapons attacks in Syria since 2013 as the parties to the conflict and their supporters push the door wide open to the use of the banned weapon, he said.
The Chemical Weapons Convention has been ratified by 192 states, making it one of the most universally supported treaties in international law. But Zeid said the rules were being flouted.
"And the world's response? Empty words, feeble condemnations, and a Security Council paralysed by the use of the veto."
Meanwhile Britain and the US agreed on Monday that a suspected poison gas attack in Syria bears the hallmarks of previous chemical weapons attacks carried out by Bashar al-Assad's Syrian government, a statement from the British foreign office said.
Following a telephone call between British foreign minister Boris Johnson and acting US Secretary of State John Sullivan, a spokeswoman said: "The Foreign Secretary and Acting Secretary of State agreed that, based on current media reports and reports from those on the ground, this attack bore hallmarks of previous chemical weapons attacks by the Assad regime.
"They reiterated their commitment to standing up for the Chemical Weapons Convention and to ensuring that those responsible for this horrific attack are held to account."
