An Iraqi officer says Iraqi forces discovered sulfur mustard, a chemical warfare agent, in Mosul alongside a cache of Russian surface-to-surface missiles, The Associated Press reports.

Iraqi special forces Brig. Gen. Haider Fadhil said French officials tested the chemical agent this week and confirmed it was sulfur mustard.

Iraqi forces showed journalists a tank of the chemical agent and a warehouse of more than a dozen surface-to-surface rockets bearing Russian inscriptions in eastern Mosul.

Fadhil said the types of rockets found lead him to suspect the Islamic State group was experimenting with the rockets to develop a way to weaponize the chemical agent.

Iraqi and US officials have repeatedly warned of IS efforts to develop chemical weapons. The extremist group has launched attacks using low-grade chemical weapons in Iraq, causing a handful of casualties.