MOSUL, Iraq (AP) — Iraqi forces discovered a mustard chemical warfare agent in eastern Mosul alongside a cache of Russian surface-to-surface missiles, an Iraqi officer said Saturday.

Iraqi and U.S. officials have repeatedly warned of Islamic State group efforts to develop chemical weapons. When Iraqi forces retook Mosul University earlier this month, they found chemistry labs they believed had been converted into makeshift chemical weapons labs.

Iraqi special forces Brig. Gen. Haider Fadhil said French officials tested the Mosul chemical this week and confirmed it was a mustard agent. Fadhil did not specify the quantity or potency of the chemical, but Iraqi soldiers were able to visit the site for about ten minutes without exhibiting any symptoms of exposure.

“We know that they were using this place to experiment with chemical weapons,” he said, referring to IS.

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Fadhil said he believes the facility was set up in the Nineveh ruins — an ancient site just over 2 kilometers (1.2 miles) from the Tigris, but removed from the city’s dense neighborhoods — to keep it a secret from Mosul residents who might be passing information to Iraqi forces and the U.S.-led coalition.

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.

The number of casualties due to IS chemical weapons is a small fraction compared to the hundreds of civilians killed in car and suicide bombings carried out by the group. Experts say that is largely due to the low grade of the weapons and the group’s lack of access to efficient delivery systems.

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The types of rockets found at the site suggest the Islamic State group was attempting to weaponize the chemical agent, Fadhil said. He added that he believed the facility was being used up until just one or two weeks ago.

Iraqi forces declared Mosul’s eastern half fully liberated, just over three months after the operation to retake the city from IS was formally launched in October.