The UN is investigating possible war crimes in eastern Mosul after 10 patients were admitted to hospital with symptoms linked to a chemical attack.
It follows an attack on a residential area along the Tigris River earlier this week in a part of Mosul liberated by Iraqi forces in January.
Hussein Qader, the deputy director of a hospital in the nearby city of Irbil, said 10 patients were admitted with symptoms of chemical weapons exposure on Thursday.
Lise Grande, UN 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.
"We call on everyone to act responsibly and to grant and ensure immediate access for appropriate parties to investigate the circumstances of the alleged attack."
The patients in the Irbil hospital are in a stable condition and will be discharged in the coming days, Mr Qader said.
Among those being treated are the children of Nazim Hamid, who said the mortar had struck their house, hitting the living room where they had been sitting.
"There was a very bad smell, it was some kind of gas," he said.
"My kids were affected, some of them were burned and some of them had difficulty breathing."
The burns were to their faces, arms and legs, he said.
Islamic State has used chemical weapons in Iraq and Syria at least 52 times, according to a report published late last year by London-based conflict monitor IHS.
At least 19 of those attacks were in and around Mosul.
While eastern Mosul was liberated two months ago, most of the city's west is still under Islamic State control.
It is the last significant urban area held by the terrorist group in Iraq, although Iraqi forces have made gains in the city's southwest during the past two weeks.