BAGHDAD — American security officials said Thursday that they were looking into a new report that Islamic State militants had used chlorine gas as a weapon against Iraqi police officers last month near Balad, north of Baghdad.

According to the accounts of the officers, fighters for the Islamic State, also known as ISIS or ISIL, set off an explosive that unleashed a mass of yellow smoke that hung close to the ground, The Washington Post reported. The Post said that hospital officials who treated the men, as well as an unnamed Iraqi Defense Ministry official, confirmed the men’s suspicion that chlorine gas had been used against them. Eleven officers were made ill, though all survived.

Unconfirmed reports of improvised bombs made with chlorine gas and used by militants have arisen from time to time since the Islamic State began seizing territory in Iraq at the beginning of the year, raising concerns that Iraq’s old chemical weapons stores had fallen into the militants’ hands.

A spokesman for the National Security Council, Alistair Baskey, said American officials were examining the new report.