A toxic chemical, almost certainly chlorine, was used "systematically and repeatedly" as a weapon in attacks on villages in northern Syria earlier this year, the global chemical weapons watchdog said Wednesday.

The Organization for the Prohibition of Chemical Weapons said that a report by a fact-finding mission it sent to Syria based its conclusion on dozens of interviews with victims, physicians, eye-witnesses and others.

The report does not apportion blame for the chlorine attacks on three villages in northern Syria, OPCW spokesman Michael Luhan said.

A copy of the full report obtained by The Associated Press says that witnesses generally linked the chlorine attacks to helicopter-borne barrel bombs, but said the helicopters were flying too high for them to see any identifying markings on the aircraft.

Both sides in Syria's conflict blame one another for using chlorine, but dropping heavy explosives from helicopters is a tactic often blamed on forces of Syrian President Bashar Assad. Human Rights Watch said in May that it had strong evidence that in April this year Syrian army helicopters dropped bombs containing chlorine on the same rebel-held villages mentioned by the OPCW report.

British Foreign Secretary Philip Hammond said the report's findings "corroborate allegations that the Assad regime is continuing to use chemical weapons in Syria, in violation of the Chemical Weapons Convention."

"That the Assad regime continues to commit such atrocities against the people of Syria is an outrage," Hammond added.

The OPCW report blamed the chemical attacks for at least 13 deaths and dozens of injuries.

The report said that witnesses described "a dense, honey wax-to-yellow hue towards the center of the cloud rising from the impact of the devices."

"In courtyards, domesticated birds and animals died, and leaves on plants facing the point of impact withered and wilted 'as autumn leaves,'" it cited witnesses as saying. "In one case, a child standing close to the impact site died later because of exposure to the toxic chemical, while showing none of the obvious physical trauma as that usually inflicted by a conventional explosive device."

Those exposed to the cloud suffered symptoms including burning or itchy eyes, a burning sensation on their skin and in their throats and a feeling of suffocation, nausea, disorientation and loss of consciousness.

"All (witnesses) described the toxic chemical smell as being very strong, irritating, and of 'chlorine,'" the report said.

It said the descriptions led it to conclude, "with a high degree of confidence" that chlorine was the chemical used. An earlier report by the mission had said it likely was used as a weapon in Syria.

Chlorine is a toxic industrial gas that is not specifically classified as a chemical weapon.

The attacks earlier this year came as Damascus and the OPCW were involved in a complex mission to remove Syria's stockpile of chemical weapons and precursor chemicals from the country. All of the poison gas, nerve agents and other chemicals declared by Syria have now been removed from the country and the most toxic parts of the arsenal destroyed.

The OPCW said the fact finding mission would continue its work as there was "a spate of new allegations" of chlorine attacks in Syria in August.