Russia's defense ministry said rebels fired grenades 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.
The United States said it had "credible information" that the account was false and that Russian and Syrian forces instead had fired tear gas.
"The United States is deeply concerned that pro-regime officials have maintained control of the attack site in its immediate aftermath, allowing them to potentially fabricate samples and contaminate the site before a proper investigation of it by the Organization for the Prohibition of Chemical Weapons," State Department spokesman Robert Palladino said in a statement.
"We caution Russia and the regime against tampering with the suspected attack site and urge them to secure the safety of impartial, independent inspectors so that those responsible can be held accountable," he said.
Russia responded to the purported attack with air raids on Idlib, throwing into question a truce reached in mid-September.
Russia, the top international backer of Assad, and the Syrian government both denied the incident, saying footage of suffering victims including children was staged.
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