The U.S. is "gravely concerned" about the Organization for the Prohibition of Chemical Weapons's findings last week of systematic and repeated chlorine attacks in northern Syria, Secretary of State John Kerry said in a written statement Sunday.
"The Assad regime must know that it will be held to account for such use in the international community," Kerry said, noting that the report raises "especially troubling concerns that continued chemical attacks on the Syrian people by the regime could occur."
The OPCW released its report on Wednesday. While it does not assign responsibility, Kerry pointed to witness accounts of helicopters being used in the attacks, which would indicate the Assad regime carried them out.
The OPCW said earlier this month that 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 organization 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 OPCW report does not apportion blame for the chlorine attacks on three villages in northern Syria.
Syrian state media and activists reported several poison gas attacks this year including one in April in the central village of Kfar Zeita that wounded scores of people. The government and the rebels blamed each other for the attack.
The Associated Press contributed to this report.