ERBIL, Iraq, Aug. 13 (UPI) -- Reports that Islamic State militants used chemical weapons in an attack in Iraq that left dozens of Kurdish fighters with breathing difficulties are credible, senior U.S. officials said.
Pentagon officials told CBS News reports that IS -- also identified as Daesh and by the acronyms ISIS and ISIL -- used mustard gas on Kurdish forces are "credible."
The terror group likely obtained the mustard agents either in Syria -- where the Assad regime has been accused of using chemical weapons against civilians -- or in Iraq.
"That makes the most sense," a senior official told The Wall Street Journal of the possibility the weapons came from Syria.
An official said mustard gas is "antiquated" and a large quantity of it must be used in order for it to be lethal.
"ISIS is a group that has demonstrated time and again its willingness to stop at nothing," the official said. "This is further evidence of that.
"The fact that it's being confirmed though, that they used it against peshmerga and the U.S. has to confront it, may be a game-changer, and that may be the real problem here: The reality that ISIS is continuing to expand and adapt and to adapt in ways that are incredibly dangerous."
The German Defense Ministry issued a statement Thursday saying 60 Kurdish fighters had experienced breathing difficulties after the attack. German soldiers are stationed near Erbil to train Kurdish soldiers to fight IS.
U.S. officials are investigating the reports, CNN said.