Twelve people are being treated for possible exposure to chemical weapons agents in Mosul, the United Nations has said, raising fears of the first chemical attack in the besieged city. The World Health Organisation (WHO), along with partners and local health authorities, has activated “an emergency response plan to safely treat men, women and children who may be exposed to the highly toxic chemical”.

It added that all 12 patients had been treated since 1 March in Erbil, the capital of Iraq’s Kurdish region, east of Mosul, with four of them showing “severe signs associated with exposure to a blister agent”. The patients were said to have been exposed to the chemical agents in the eastern side of Mosul, where Islamic State militants have been overcome by an offensive by US-backed Iraqi forces.

Iraqi forces captured the eastern part of the city in January after 100 days of fighting, and launched their attack on the districts that lie west of the Tigris river on 19 February. The eastern side remains within reach of the militants’ rockets and mortar shells.

The International Committee of the Red Cross (ICRC) said on Friday that five children and two women were receiving treatment for exposure to chemical agents. The ICRC statement did not say which side used the chemical agents, which caused blisters, redness in the eyes, irritation, vomiting and coughing.

The UN’s humanitarian coordinator in Iraq, Lise Grande, called for an investigation. “This is horrible. If the 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,” she said in a statement.

Defeating Isis in Mosul would crush the Iraqi wing of the caliphate declared by the group’s leader, Abu Bakr al-Baghdadi, in 2014, over parts of Iraq and Syria.

Thousands of civilians fled in the early hours of Friday as Iraqi forces regained control of the Wadi Hajar district in western Mosul from Isis. Since the Mosul offensive began about two weeks ago, more than 28,000 people have been displaced by the fighting, the UN said.

UN spokesman Stéphane Dujarric said about 750,000 people remain trapped in western Mosul, either sheltering from the fighting or waiting to escape. “We’re deeply concerned with their wellbeing and safety and their access to vital resources.”

Nahla Ahmed, 50, fled her home in the Shuhada neighbourhood, which remains controlled by Isis. “All the families were hiding behind a wall. We gave the children Valium so they wouldn’t cry and [the IS fighters] wouldn’t catch us,” she said.

Muthana Younis also escaped from Mosul. “We waited until all the Isis fighters left the streets. There were mortars and we could hear gunfire, but we had to flee – we had run out of food.”

