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

“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 Daesh 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.

The ICRC, however, did not disclose which side used the chemical agents that resulted in blisters, redness in the eyes, irritation, vomiting and coughing.

The commander of the Iraqi rapid response forces, Captain Sa'adon Khaled al-Ramadani, said on Thursday that the extremists lobbed a barrage of Katyusha rockets containing chlorine gas on al-Maliyah, Nabi Yunus and al-Faisaliyah neighborhoods of Mosul.

A number of people were reportedly transferred to medical centers and hospitals after suffering severe poisoning with the toxic gas.

Noureddin Qablan, the deputy chief of the Nineveh Provincial Council, also announced that a woman and her two children suffered skin burns and dyspnea after rockets laced with mustard gas slammed into Mosul's al-Samah and Northern Karaj neighborhoods.

Daesh has launched several chemical attacks in Iraq and neighboring Syria. 

On February 19, Iraqi government troops and fighters from Popular Mobilization Units – commonly known by the Arabic name Hashd al-Sha’abi – mounted a new offensive to liberate western Mosul.

Iraqi forces took full control of Mosul’s eastern part on January 23.

Mosul is considered the last urban stronghold of the Daesh terror group in Iraq.