Chlorine gas was used in attacks on northern Syria earlier this year, according to the global chemical weapons watchdog.

The Organization for the Prohibition of Chemical Weapons (OPCW) said on Wednesday it had "compelling confirmation" that a toxic chemical, likely pure or mixed chlorine, had been used "systematically and repeatedly" as a weapon in parts of the country where anti-government rebels have been clashing with Syrian President Bashar al-Assad's troops.

The OPCW has been overseeing the disassembling and destruction of Syria's chemical weapons stockpile after Assad's government signed the Chemical Weapons Convention (CWC) in October 2013 following a chemical attack in August that year that killed as many as 1,400 people. However, it announced in late April that it would launch a fact-finding mission to investigate the reported use of chlorine gas.

Chlorine was used in attacks on three villages, the mission concluded, after conducting dozens of interviews with victims, doctors and eyewitnesses, as well as reviewing medical records, video and other documentation.

"The descriptions, physical properties, behavior of the gas, and signs and symptoms resulting from exposure, as well as the response of patients to the treatment, leads the FFM [fact finding mission] to conclude with a high degree of confidence that chlorine, either pure or in mixture, is the toxic chemical in question," the OPCW said.

A convoy of inspectors from the mission came under attack in Syria in May while traveling to the rebel-held of Kafr Zita, site of one of the alleged incidents.

The OPCW noted that there had been a reduction in reported chlorine attacks since the fact finding mission had begun its work, but that a number of new allegations had surfaced in August. These too, it added will be investigated.

A report released by Human Rights Watch (HRW) in May concluded, based on witness interviews as well as video and picture analysis, that Assad's forces had used bombs containing chlorine gas on the villages.