Chlorine was used in an attack on the Syrian city of Douma that killed dozens of civilians in April, the world’s chemical weapons watchdog has found.

A preliminary report by the Organisation for the Prohibition of Chemical Weapons (OPCW) said it found “various chlorinated organic chemicals” in two locations, but there was no evidence nerve agents were used.

Dozens of civilians were killed in the attack on the rebel-held town in the Eastern Ghouta region, near the country's capital Damascus.

Footage of young children foaming at the mouth following the incident elicited horrified reactions from people around the world.

The OPCW’s findings confirm reports by medics who said they treated 500 patients “exhibiting signs and symptoms consistent with exposure to toxic chemicals”.

Western governments blamed President Bashar al-Assad’s regime for the attack and US, British and French warplanes carried out strikes against government military targets in retaliation.

The Syrian government denies carrying out any attacks with chemical weapons, but the OPCW has documented systematic use of banned munitions in Syria’s civil war by both the regime and Isis.

The OPCW has not attributed blame for the banned munitions in the April attack, but its investigation into the use of chemical weapons is continuing.

The chemical weapons watchdog was initially barred from accessing the site of the attack, but workers were granted access in mid-April.

Douma chemical attack: Syria war in pictures Show all 13 1 /13 Douma chemical attack: Syria war in pictures Douma chemical attack: Syria war in pictures A child receiving oxygen through respirators following an alleged poison gas attack in the rebel-held town of Douma AP Douma chemical attack: Syria war in pictures A child is treated in a hospital in Douma White Helmets Douma chemical attack: Syria war in pictures An image grab taken from a video released by the Douma City Coordination Committee shows unidentified volunteers spraying a man with water at a make-shift hospital following an alleged chemical attack AFP/Douma City Coordination Committee Douma chemical attack: Syria war in pictures People cover their faces after an alleged chemical attack in Douma EPA Douma chemical attack: Syria war in pictures A medical worker giving a toddler oxygen through respirators following an alleged poison gas attack in the opposition-held town of Douma. Syrian Civil Defense White Helmets via AP Douma chemical attack: Syria war in pictures An image grab taken from a video released by the Douma City Coordination Committee shows unidentified volunteers spraying a girl with water at a make-shift hospital following an alleged chemical attack AFP/Douma City Coordination Committee Douma chemical attack: Syria war in pictures A child is treated in a hospital in Douma White Helmets/Handout via Reuters Douma chemical attack: Syria war in pictures Injured victims of an alleged chemical attack in Douma EPA Douma chemical attack: Syria war in pictures An image grab taken from a video released by the Douma City Coordination Committee shows unidentified volunteers spraying a man with water at a make-shift hospital following an alleged chemical attack AFP/Douma City Coordination Committee Douma chemical attack: Syria war in pictures A Syrian man mourns after an alleged chemical attack Alamy Douma chemical attack: Syria war in pictures People stand in front of damaged buildings, in the town of Douma, the site of a suspected chemical weapons attack, near Damascus, Syria AP Photo/Hassan Ammar Douma chemical attack: Syria war in pictures Rubble lines a street in Douma, at the site of a suspected chemical weapons attack AP Douma chemical attack: Syria war in pictures Rubble fills a street in Douma, the site of a suspected chemical weapons attack, near Damascus AP

Inspectors visited two sites where they interviewed witnesses and took samples, which were taken to their lab in the Netherlands before being forwarded to affiliated national labs for testing.

Two samples recovered from gas cylinders at the scene tested positive for chlorine, the report said.

In the aftermath of the deadly attack, US media quoted unnamed officials who said both chlorine and sarin were used in the attack.

A UN investigation said evidence pointed to the use of chlorine, but that some people had exhibited symptoms indicating they were exposed to a nerve agent.

Yet the OPCW report said “no organophosphorous nerve agents or their degradation products were detected”.

Civilians in Eastern Ghouta, an agricultural area, lived under seige for five years until a Russia-backed offensive drove the rebels out - reportedly killing 1,700 men woman and children in the process.