U.S. Investigating ISLAMIC STATE's Use Of Chemical Weapons Against Kurdish Peshmerga Fighters

Mustard gas used against Kurdish fighters in Iraq. It is unclear if the gas was leftover from Saddam Hussein stockpiles or if they were brought over from Syria. CNN's Barbara Starr reports:

IS suspected of chemical arms attack on Kurds in Iraq--- 

"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. U.S. officials told local media they believed it was mustard agent. IS has previously been accused of using chlorine gas against Kurdish fighters.

U.S. officials quoted in the Wall Street Journal said IS could have obtained the mustard agent in neighboring 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 defense 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.