The UN says the recent chemical attack in Mosul, believed to be the first by the ISIS terrorist group in the northern Iraqi city, could amount to a war crime.

Axar.az reports that, “This is horrible. 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,” the UN humanitarian coordinator in Iraq, Lise Grande, said in a statement, demanding an investigation.

The World Health Organization said on Saturday that 12 people were receiving treatment in the city of Erbil, located approximately 350 kilometers north of the capital Baghdad, for possible exposure to chemical weapons agents in Mosul, adding that four of them showed “severe signs associated with exposure to a blister agent.”

The UN body said that it was working with medical authorities in Iraq's semi-autonomous northern region of Kurdistan, and has activated “an emergency response plan to safely treat men, women and children who may be exposed to the highly toxic chemical.”

It is not immediately clear who launched the attack, but media reports suggest that mortar shells came from the western part of Mosul, which is still under IS control.

The report came a day after the International Committee of the Red Cross (ICRC) announced that five children and two women were receiving treatment for exposure to chemical agents in Mosul.