Nine Kurdish soldiers, known as Peshmerga, were admitted to Azadi Teaching Hospital in the city of Dohuk last Friday with symptoms including vomiting, nausea, shortness of breath and itching, the director of the hospital, Dr Afrasiab Mussa Yones, told The Associated Press.
He said that the symptoms suggested that chlorine had been used, but that further analysis was needed. Yones said he would send samples taken from the soldiers' clothes for analysis.
"One of the mortar rounds landed near my position and there was a lot of smoke," said Col. Lukhman Kulli Ibrahim of the 8th Peshmerga brigade based in Sinjar. He said he "fell down immediately and went unconscious."
He said that after coming to, "I felt burning in my eyes, I struggled to breathe, had a headache and a burning in my chest."
The Organization for the Prohibition of Chemical Weapons confirmed earlier this week that IS used mustard gas on Kurdish forces last August.
Chlorine is an easily obtainable chemical element that is widely used in water purification. It was first used as a weapon in the First World War. When it reacts with water in the lungs it forms hydrochloric acid, a potentially lethal irritant.