The international chemical weapons watchdog has said it has “reasonable grounds to believe” that the Syrian air force was responsible for a chlorine bomb that was dropped on a town in the country’s rebel-controlled Idlib region three years ago.

In a report published on Monday, the investigative arm of the Organisation for the Prohibition of Chemical Weapons attributed the attack, which resulted in 12 casualties, to Syria’s Tiger Forces, an elite unit under the control of the air intelligence directorate.

Experts said the report was just the latest in a growing accumulation of evidence that President Bashar al-Assad’s forces, backed by Russia and Iran, used banned chemical weapons during the decade-long civil war. Both Damascus and Moscow have previously denied these claims.

According to the OPCW investigation team, a military helicopter under the command of the Tiger Forces dropped at least one chlorine cylinder on the northern Syrian town of Saraqeb in February 2018. At the time, the town was held by rebel forces.

“The cylinder ruptured and released chlorine over a large area, affecting 12 named individuals,” the watchdog said in a statement. The victims, all of whom survived, were treated for symptoms consistent with chemical poisoning including shortness of breath, skin irritation, chest pain and coughing.

The watchdog’s investigation and identification team (IIT), which conducted the report, reviewed more than 400 gigabytes of material including satellite imagery, as well as obtaining 44 statements from more than 30 witnesses and analysing samples from the scene.

The team was established in 2019 after Russia blocked the extension of an existing investigatory task force set up jointly by the UN and OPCW to probe the use of chemical weapons during Syria’s civil conflict. In its first report last year, the IIT implicated Assad’s forces in another attack in March 2017, in which bombs containing chlorine and sarin nerve gas were dropped on a village in Syria's Hama region.

The OPCW does not have the power to assign criminal responsibility for attacks, but the findings will be shared with the watchdog’s member states and the UN.

Responding to the report, Josep Borrell, the EU’s foreign policy chief, said the use of chemical weapons was a violation of international law.

“It is now up to the international community to duly consider the reports and take appropriate action,” he said in a statement.

Tobias Schneider, an expert in Middle Eastern security at Berlin’s Global Public Policy Institute, said that victims of the attacks had been trying to hold the regime to account for years without success.

“This report is a form of truth-seeking, it’s a reliable investigation that will hold up in court, so for lawyers who are trying to bring war crimes cases, this is their evidence base,” he said. “For the first time we can reliably say the international investigators have found the Tiger Forces directly responsible for this attack, it’s no longer pure conjecture.”

Bassam Barabandi, a former Syrian diplomat who has since defected, said the confirmed involvement of Tiger Forces helps to implicate Moscow directly in the use of chemical weapons, as Russia is Assad’s main foreign military backer.

“This report should help the US and EU countries to put more pressure on Russia to say, enough is enough,” he said.