There is intense fighting in Mosul as Iraqi troops attempt to recapture the last ISIL-held enclave in the city.
The area targeted takes in the Old City and three other districts.
Iraqi fighter jets dropped leaflets, urging residents to take cover.
The battle to recapture the city, now in its eighth month, has taken much longer than expected, with militants putting up stiff resistance and mixing in with civilians.
The fall of Mosul would mark the end of the Iraqi half of the so-called caliphate that ISIL wanted to create.