Reports claim ISIS fighters used mustard agent in an attack against Kurdish Peshmerga in northern Iraq. Photo: Wikimedia Commons.

The U.S. is investigating what it believes are “credible” reports that ISIS fighters used mustard agent in an attack against Kurdish Peshmerga this week, causing several of them to fall ill, U.S. officials working in at least three separate parts of the Obama administration said Thursday.

All of them strongly emphasized more intelligence is being gathered on exactly what may have happened near the town of Makhmour in northern Iraq. While there have been accounts posted in social media about the incident, the officials said they have independent information that strongly led them to assess there was a use of chemical weapons. The officials would not tell CNN what evidence led them to this belief.

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