THE HAGUE, Netherlands — Insurgents in Syria used sulfur mustard in August, likely killing an infant, the Organization for the Prohibition of Chemical Weapons said Friday.

The OPCW statement was the first official confirmation that sulfur mustard, often called mustard gas, has been used by insurgents in Syria.

An OPCW source said Thursday that inspectors found evidence of sulfur mustard use by insurgents in the town of Marea on Aug. 21. Syria’s brutal civil war has already killed 250,000 people and forced millions to flee.

The inspectors based their conclusions on samples they analyzed and interviews with two survivors and the physicians who treated them.

Sulfur mustard is an outlawed chemical weapon that attacks the skin, eyes, lungs and other internal organs.

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