The report is the second by an investigations team set up by the Organisation for the Prohibition of Chemical Weapons (OPCW), which has the new power to apportion blame for attacks.

The OPCW said in a statement that the Investigations and Identification Team "concludes that units of the Syrian Arab Air Force used chemical weapons in Saraqib on 4 February 2018".

"The report reached the conclusion that there are reasonable grounds to believe that, at approximately 21:22 on 4 February 2018, a military helicopter of the Syrian Arab Air Force under the control of the Tiger Forces hit eastern Saraqib by dropping at least one cylinder," the OPCW report said.

"The cylinder ruptured and released chlorine over a large area, affecting 12 named individuals."

OPCW investigators interviewed witnesses, analysed samples collected at the scene, reviewed symptoms reported by victims and medical staff and examined satellite imagery to reach their conclusions, the organisation said.

The team issued its first report a year ago, in which it said Syrian President Bashar al-Assad's air force used the nerve agent sarin and chlorine in two attacks on the village of Lataminah in March 2017.

- Syria sanctions mooted -

Despite strong objections by Damascus and its ally Moscow, OPCW member states voted in 2018 to set up the team to attribute blame for attacks. Previously the watchdog could only say if chemical attacks had happened or not.

Syria has continued to deny the use of chemical weapons and insists it has handed over its weapons stockpiles under a 2013 agreement, prompted by a suspected sarin attack that killed 1,400 in the Damascus suburb of Ghouta.

OPCW states will vote later this month on whether to impose sanctions on Syria, including the suspension of its voting rights in the organisation, over its failure to comply with its rules.

The OPCW's governing body has told Syria it must declare all details about the facilities used to produce the sarin and chlorine used in the 2017 attacks.

Western powers have expressed concern that Damascus has failed to declare and destroy all its chemical weapons under the 2013 deal.

The OPCW chief said in March there were still gaps and inconsistencies in Syria's reports to the body.

The United Nations said in March that Damascus had for years failed to reply to a series of 19 questions about its weapons installations, which could have been used to stock or produce chemical weapons.