Mohamed Ali Alhakim, Iraq's ambassador to the United Nations, has said there is "no evidence" that ISIL fighters have used chemical weapons in Mosul, where they are battling an offensive by US-backed Iraqi forces.

His comments on Friday came nearly a week after medical workers said that several people, including women and children, had been treated for possible exposure to chemical weapons agents in Mosul since the start of the month.

Alhakim said he spoke with officials in Baghdad and "there was really no evidence that Daesh has used this chemical weapon".

Daesh is another name for ISIL, which stands for Islamic State of Iraq and the Levant, and is also known as ISIS.

Alhakim said Iraq had been in contact with the Hague-based Organisation for the Prohibition of Chemical Weapons (OPCW) which could dispatch a team of experts in the event of a suspected toxic gas attack.

READ MORE: UN says alleged Mosul chemical attack amounts to war crime

The US Department of Defense has said that ISIL fighters developed rudimentary chemical weapons, such as sulphur mustard at the University of Mosul.

Iraqi forces who retook the campus earlier this year said that parts were repurposed as weapons production facilities.

"There are chemical weapons there," Brigadier-General Sami al-Aradi told Al Jazeera in January. "We've cordoned off the laboratories until we can find out exactly what ISIS were doing."

No formal investigation

The World Health Organization, a United Nations agency, said in a statement on March 3 that 12 people had been treated for possible exposure to chemical weapons agents in Mosul since the start of the month.

That number has now increased to 18.

"Of these, four patients are showing severe signs associated with exposure to a blister agent. WHO and partners are working with health authorities in Erbil to provide support in managing these patients," the statement said, adding that the use of chemical weapons would amount to a "war crime".

Al Jazeera's Stefanie Dekker visited the hospital in Erbil where the patients were being treated.

On March 4, she said the hospital manager was "certain that some form of chemical gas has been used".

"The manager of the hospital told us when these patients came in, the emanated a very strong odour, which is one of the side effects of being affected by these chemical gases," Dekker said.

So far, no formal investigation into the use of chemical weapons has been launched.

The International Committee of the Red Cross (ICRC) said on March 3 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 that caused blisters, redness in the eyes, irritation, vomiting and coughing.

Iraqi forces captured the eastern side of Mosul in January after 100 days of fighting and launched their attack on the districts that lie west of the Tigris River on February 19.

The eastern side of the city remains within reach of ISIL fighters' rockets and mortar shells.