Militants from Islamic State (IS) are suspected of using chemical weapons in an attack against Kurdish forces in northern Iraq, officials say.
German officials said Kurdish troops suffered breathing difficulties after an attack near the city of Irbil earlier this month.
They did not say what may have been used. US officials told local media they believed it was mustard agent.
IS has previously been accused of using chlorine gas against Kurdish fighters.
US officials quoted in the Wall Street Journal said IS could have obtained the mustard agent in neighbouring Syria.
The Syrian government has previously said that all its stockpiles of such weapons have been destroyed.
Former Iraqi leader Saddam Hussein also used weapons such as mustard agent against the Kurds and against Iran.