Iraq's Federal Police say Daesh shelled government forces with chemical weapons agents in Urouba and Bab Jadid districts.
Iraqi police on Sunday accused Daesh of using chemical weapons against their forces in Mosul.
Mosul, Iraq's second biggest city, was captured by Daesh in 2014, but government forces have retaken much of it during an operation that has lasted six months.
Officers in Iraq's Federal Police said that Daesh shelled government forces with chemical weapons agents in the Urouba and Bab Jadid districts on Saturday.
The attack caused only minor wounds, the force said in a statement, without giving more details.
The UN said last month that 12 people, including women and children, had been treated for possible exposure to chemical weapons agents in Mosul.
But Iraq's UN ambassador, Mohamed Ali Alhakim, said days later there was no evidence for that.
New push against Daesh
Iraq's Federal Police, one of several forces attacking the group, said it had made a new push against the group holed up in the Old City, where tanks and heavy vehicles are not able to operate because of its narrow streets.
The front has hardly moved for over a month.
Federal Police forces moved 200 metres (yards) deeper into the Old City, getting closer to Al Nuri mosque, a statement said.
The mosque is highly symbolic because it was there that Daesh leader Abu Bakr al-Baghdadi declared himself head of a self-proclaimed caliphate.
Troops have had the centuries-old mosque with its leaning minaret in their sights since last month.
A captain in the Federal Police said Sunday's advance had started in the early morning with troops fighting the militants house-to-house.
"Daesh suicide motorcycles now are their favourite weapon inside the Old City," he said.
Iraqi government forces, backed by US advisers, artillery and air support, have cleared the east of Mosul and half of the west and are now focused on the Old City.
Some 400,000 people are trapped in the area and more than 300,000 have fled fighting since the operation started in October, officials say.