The United States and Britain accused Russia on Friday of fabricating a story about chemical weapons use by Syrian rebels and warned Moscow against undermining a shaky truce.
Russia's defense ministry said rebels fired weapons containing chlorine on November 24 on the regime-held city of Aleppo, with Syrian state media reporting that around 100 Syrians were hospitalized for breathing difficulties.
Russia responded to the purported attack with air raids on Idlib, the latest major stronghold of rebels and jihadists battling President Bashar al-Assad, throwing into question a truce reached in mid-September.
The United States said it had "credible information" that the chlorine account was false and that Russian and Syrian forces instead had fired tear gas.
In a similar statement, Britain said it was "highly unlikely" that chlorine or the opposition were involved in the incident.
Russia's embassy in Washington hit back on Facebook.
"The Russian Defense Ministry does not rule out that the US Department of State's allegations about the recent toxic chemicals attack in Syria's Aleppo are aimed at distracting the public attention from the crimes of the US aviation in the east of the Middle Eastern country," the post said.
Russia's allegations over the latest incident come amid elusive efforts to find a political solution to Syria's civil war, which has killed more than 360,000 people and displaced millions.
Negotiators from Russia and fellow Assad ally Iran met last week with opposition supporter Turkey in Kazakhstan's capital Astana, making no apparent headway in a UN-backed goal of setting up a constitutional committee by the end of the year.
But Russia and Turkey agreed to keep working to preserve the U-shaped buffer zone around Idlib, which is keeping pro-government forces out of the region.