Volunteers in the northern Syrian city of Aleppo wear gas masks on Sept. 15, 2013, during a class on how to respond to a chemical attack. (JM Lopez/AFP/Getty Images)

Inspectors have conclusively identified mustard gas as the toxic agent used in an attack by insurgents in northern Syria this summer, according to a statement released Friday by an international chemical-weapons watchdog.

The findings by the Organization for the Prohibition of Chemical Weapons are the first confirmation that non-state actors have used mustard gas in Syria’s four-year-old conflict. They also are the first confirmation of the use of the toxin since the Syrian government agreed two years ago to destroy its stockpile of chemical weapons. Since then, Syrian President Bashar al-
Assad’s forces have been accused of carrying out many attacks using chlorine gas, a choking agent.

The OPCW did not specify which group used mustard gas in the attack, which the organization said killed at least one person, an infant, on Aug. 21 in the village of Marea. Doctors and an aid organization in the village north of Aleppo blamed the attack on Islamic State militants who for months have been battling rebel groups in the area.

The use of mustard gas by either the Islamic State or rebels, or both, is a significant escalation in a conflict that has killed 250,000 people and displaced millions.

“In this case, the team was able to confirm with utmost confidence that at least two people were exposed to sulfur mustard, and that it is very likely that the effects of this chemical weapon resulted in the death of an infant,” the OPCW statement said.

A spokeswoman for the National Security Council said U.S. officials are “very concerned” about the OPCW findings. The spokeswoman said the United States is “continuing to investigate these allegations very closely and to be proactive about the threat from chemical weapons, or other similar threats.”

Mustard gas, formally known as sulfur mustard, is a banned chemical weapon that can cause severe blistering of the skin and mucous membranes, as well as other symptoms such as wheezing and severe itching. After the Marea attack, doctors in the area reported dozens of residents exhibiting such symptoms.

It is unclear how non-state forces in Syria could obtain and weaponize chemical toxins such as mustard gas.

The Assad government says its 1,300-ton stockpile of chemical weapons was destroyed as part of an agreement between the United States and Russia. Weapons inspectors and U.S. intelligence officials question that assertion, suspecting that the Syrian government concealed some of the original stock.

In its statement, the OPCW said another team of investigators found that chlorine gas had “likely” been involved in an attack in the northwestern province of Idlib. The rebel-held province is regularly targeted by government aircraft that indiscriminately attack with chlorine-gas weapons, opposition groups allege.

The OPCW also said its investigators could not find evidence that rebels in August targeted government forces in a Damascus suburb with chemical agents.

Karen DeYoung in Washington contributed to this report.

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