A US official on Thursday deemed it "plausible" that the Islamic State group used mustard gas against Iraqi Kurd fighters this week.
"Based on previous information, we found these reports plausible," the official told AFP, referring to a Wall Street Journal-story citing US and German sources that said the extremist group had used the banned chemical weapon.
For its part, the Pentagon said on Thursday it is "seeking additional information" about the alleged attack.
"We continue to take these and all allegations of chemical weapons use very seriously," said Pentagon spokesman Navy Captain Jeff Davis.
According to the Wall Street Journal, German officials said approximately 60 Iraqi Kurd fighters, known as Peshmerga, reportedly "suffered injuries to their throats consistent with a chemical attack while fighting Islamic State" on Wednesday.
US officials confirmed to the US newspaper that mustard gas was likely used in the attack and that the jihadists could have obtained the chemical weapons in Syria. US officials did not confirm to AFP when the attack had taken place.
Peshmerga fighters told AFP on Thursday that they had been the target of a chemical attack on Tuesday. They suggested rockets filled with chlorine gas were to blame and did not mention mustard gas.
The German Defense Ministry has said that Iraqi and US specialists are on their way to the scene of the attack to investigate. Mustard gas is an asphyxiant that has been banned in war by the UN since 1993.