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 hospitalised 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 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.
He said that Russia and Syria were "using it as an opportunity to undermine confidence in the ceasefire in Idlib." In a similar statement, Britain said it was "highly unlikely" that chlorine or the opposition were involved in the incident.
"It is likely that this was either a staged incident intended to frame the opposition, or an operation which went wrong and from which Russia and the regime sought to take advantage," a Foreign Office spokesperson said, also backing an investigation by the OPCW, the international chemical weapons watchdog.
Western powers, the United Nations and human rights groups have repeatedly pointed to chemical attacks by Assad's forces.
A sarin gas attack in April 2017 in the town of Khan Sheikhun killed 83 people, according to the UN.
US President Donald Trump replied by ordering 59 cruise missiles to strike a Syrian air base, a reversal from his predecessor Barack Obama's controversial reluctance to respond militarily.
Russia, the top international backer of Assad, and the Syrian government both denied the incident, saying footage of suffering victims including children was staged.
The official, speaking on condition of anonymity, said that witnesses did not report the odor of chlorine that is characteristic of such attacks.
"Technical analysis of videos and images of munition remnants indicate the mortars portrayed in Russian media are not suitable for delivering chlorine," the official 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.
Jan Egeland, in a press conference last week before he stepped down as the head of the UN Humanitarian Task Force for Syria, warned that the flare-up amounted to "a gigantic powder keg in the middle of three million civilians." (AFP) IND
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