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.

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

The report does not apportion blame for the chlorine attacks on three villages in northern Syria.

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.

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