Alleged chemical weapons attacks in Iraq would constitute a war crime if confirmed, the United Nations said in a statement on Saturday.

“This is horrible,” Lise Grande, the U.N. humanitarian coordinator in Iraq said. “If the alleged use of chemical weapons is confirmed, this is a serious violation of international humanitarian law and a war crime, regardless of who the targets or the victims of the attacks are.”

“There is never justification — none whatsoever — for the use of chemical weapons,” she added.

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The warning came after doctors in Irbil reported seeing patients with symptoms consistent with exposure to chemical weapons, including burns on their faces, arms and legs, the Associated Press reported.

The attack reportedly happened in Eastern Mosul earlier this week. Eastern Mosul has been declared fully liberated from the control of the Islamic State in Iraq and Syria (ISIS), and Iraqi forces, backed by U.S.-led coalition airstrikes, are working to retake the city’s western half from militants.

The operation to retake Mosul began in October,and, if successful, would bring down ISIS’s last urban stronghold in Iraq.

If confirmed, the recent allegations of chemical weapons use by ISIS wouldn’t be the first. The extremist group has used such weapons at least 52 times in Iraq and Syria, the AP reported, citing a report by the London-based research company IHS conflict monitor.

The U.N.’s ban on the use of chemical weapons has been in effect since 1997.