Poisonous chlorine gas was used as a weapon on a rebel-held Syrian town earlier this year, tests conducted by an international chemical weapons watchdog show.

Investigators from the Organisation for the Prohibition of Chemical Weapons (OPCW) used laboratory tests to determine Saraqib in Idlib province was targeted by chlorine released from cylinders in an attack in February.

As per standard OPCW policy, Wednesday’s latest report on the systemic use of banned munitions in the Syrian civil war did not speculate as to who was behind the attack.

Syria civil war: Footage shows children treated following chemical weapons attack in Douma

Witnesses on the ground, however, told monitors and journalists the chlorine was dropped by illegal barrel bombs from a helicopter. The only fighting force in Syria known to use low-flying helicopters in bombing raids is President Bashar al-Assad’s government.

The OPCW report used samples from soil, canisters and impact sites to determine “chlorine was released from cylinders by mechanical impact in the Al Talil neighbourhood of Saraqib”.

A total of 11 people were treated for breathing difficulties, vomiting and unconsciousness in the attack on 4 February – symptoms consistent with exposure to the toxic gas.

Syria bombings: US, UK and French military air strikes wreckage Show all 13 1 /13 Syria bombings: US, UK and French military air strikes wreckage Syria bombings: US, UK and French military air strikes wreckage The wreckage of the Scientific Studies and Research Centre in the Barzeh district, north of Damascus, which was targeted by the US, UK and France air strikes. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage A Syrian soldier films the damage of the Syrian Scientific Research Center surrounded by papers and rubble. AP Syria bombings: US, UK and French military air strikes wreckage Firefighrers extinguish smoke that rises from the damage. The Pentagon says none of the missiles filed by the U.S. and its allies was deflected by Syrian air defenses, rebutting claims by the Russian and Syrian governments. AP Syria bombings: US, UK and French military air strikes wreckage The wreckage of part of the Scientific Studies and Research Centre compound . AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Part of a building collapsing, surrounded by the wreckage. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Further damaged to the Scientific Studies and Research Centre compound. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Lt. Gen. Kenneth McKenzie, the director of the Joint Staff at the Pentagon, also says there also is no indication that Russian air defense systems were employed early Saturday in Syria. AP Syria bombings: US, UK and French military air strikes wreckage A Syrian soldier sprays water on the wreckage. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Syrian state news agency SANA reported several missiles hit a research centre in Barzeh, north of Damascus, "destroying a building that included scientific labs and a training centre". AFP/Getty Syria bombings: US, UK and French military air strikes wreckage A Syrian soldier films the damage. AP Syria bombings: US, UK and French military air strikes wreckage Damage to the Scientific Research Center building that was hit by the strikes. EPA Syria bombings: US, UK and French military air strikes wreckage The Scientific Studies and Research Centre was one of the targeted buildings by the US, UK and France. AFP/Getty Syria bombings: US, UK and French military air strikes wreckage Further damage to the centre. EPA

Mr Assad has repeatedly denied his government has any chemical weapons stocks after agreeing to give them up to the OPCW in 2013. The opposition has long accused the regime of holding back some of its supply.

The OPCW has said it believes Mr Assad’s government is responsible for at least three chlorine and sarin weapons attacks that have targeted civilians since then – claims the president also denies. Rebels were found to have used sulfur mustard once, on a small scale.

The watchdog is due to release its initial findings into the deaths of at least 70 people in a suspected chlorine attack in April in the then rebel-held Damascus enclave of Douma.

The incident prompted missile strikes on Syrian government weapons research facilities from the US, UK and France.

The report is expected by the end of May.