The global chemical weapons watchdog has "reasonable grounds to believe" that Syria's air force dropped a chlorine bomb on a residential neighbourhood in the rebel-controlled Idlib region in February 2018, a report released on Monday said.
Syria and its military ally Russia have consistently denied using chemical weapons during President Bashar Al Assad's decade-old conflict with rebel forces, saying any such attacks were staged by opponents to make Damascus look like the culprit.
The report by the investigative arm of the Organisation for the Prohibition of Chemical Weapons said no one was killed when the cylinder of chlorine gas, delivered in a barrel bomb, hit Al Talil neighbourhood in the city of Saraqib in February 2018.
However, a dozen people were treated for symptoms consistent with chemical poisoning, including nausea, eye irritation, shortness of breath, coughing and wheezing, it said.
Chlorine is not an internationally banned toxin, but the use of any chemical substance in armed conflict is banned under the 1997 Chemical Weapons Convention, the implementation of which is overseen by the OPCW from The Hague.
In April 2020, the OPCW's investigation and identification team concluded that Syrian warplanes and a helicopter had dropped bombs containing chlorine and sarin nerve gas on a village in Syria's Hama region in March 2017.
The latest report by the team also implicated Syrian government forces. It concluded that "there were reasonable grounds to believe that at least one cylinder filled with chlorine was dropped from a helicopter of the Syrian Arab Air Forces, belonging to the Tiger Forces".
The Tiger Forces are an elite Syrian military unit generally used in offensive operations in the war, which has largely subsided after Mr Al Assad wrested back most territory with Russian and Iranian support.
"All elements indicated the presence of Tiger Forces in the vicinity of Saraqib. They found that a helicopter was flying just above the bombed area at the moment of the gas release," a summary of the OPCW report said.
It said that samples collected from the scene were examined and other possible means of chlorine contamination considered, but the OPCW team found nothing to indicate that the incident was staged by the Assad regime’s adversaries.
The team identified individuals believed to be involved in the alleged attack but did not release their names.
UN Secretary General Antonio Guterres received the report on Monday and was "deeply concerned by its findings", said a UN spokesman.
The UN chief "condemns the use of chemical weapons and reiterates his position that the use of chemical weapons anywhere, by anyone, and under any circumstances is intolerable, and impunity for their use is equally unacceptable," he added.
"It is imperative to identify and hold accountable all those who have used chemical weapons."
Louis Charbonneau, a UN expert with the New York-based campaign group Human Rights Watch, said the culprits for the Saraqib attack must be “held to account” and bashed Russia for hampering the investigation.
Between 2015 and 2017, a joint United Nations-OPCW team known as the Joint Investigative Mechanism found that Syrian government troops had used sarin and chlorine barrel bombs on several occasions, while ISIS militants were found to have used mustard gas.