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.
A US defence official told the BBC that it was aware of the reports and was looking into them, adding: "It is plausible."
Mustard agent, or sulphur mustard - more commonly referred to as mustard gas - causes blistering of the skin, eyes and respiratory tract.
The German defence ministry, which is helping to train Kurdish forces, said that US and Iraqi experts were on their way to the area "to find out what happened".
A ministry statement said that about 60 Kurdish fighters had suffered breathing problems as a result of the attack.
"We have indications that there was an attack with chemical weapons," a ministry spokesman told AFP news agency.
Kurdish security officials said the IS attack happened near the town of Makhmour, south-west of Irbil.
Earlier this year, the autonomous Kurdish government in northern Iraq said it had evidence that IS had used chlorine in a car bomb attack.
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