Islamic State militants set fire to sulfur stocks at a factory south of Mosul, the U.S. military said Saturday, creating a plume of noxious smoke that has drifted over a base with U.S. troops involved in the Iraqi offensive to retake the city and forcing some troops to put on gas masks as a precaution.

People in the area affected by the smoke said it was difficult to breathe, burned their eyes and stung their noses and throats when they inhaled it, and burned any exposed wet skin.

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Sulfur dioxide from burning stocks is highly toxic and can be lethal. It can also quickly cause shortness of breath and coughing, according to a chemical fact sheet from Sultran, a Canadian rail company that ships large quantities of sulfur.

Militants set the residue alight at the Mishraq sulfur plant on Thursday as a tactical measure to slow Iraqi military advances in the offensive to recapture Mosul—Islamic State’s last remaining stronghold in Iraq. The toxic cloud mixed with choking black smoke already filling the air from oil-well fires started two months ago and still burning in the town of Qayara. The combination affected the nearby U.S. base as shifting winds blew the smoke toward the troops.

“Daesh ignited toxic sulfur residue stored at al-Mishraq in an attempt to disrupt the ISF [Iraqi Security Forces’] advance,” said Col.John Dorrian, a spokesman for the U.S.-led coalition in Iraq, using another name for Islamic State. He added that the military is now assessing the risk to U.S. troops because of the multiple fires.

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