Near Mosul, Iraq (CNN)Iraqi-led forces engaged Thursday in the most intense fighting yet in the battle to liberate the city of Mosul from two years of brutal ISIS rule, on day four of an offensive that's been met with fierce resistance from ISIS fighters.
Concerns for civilians
Peshmerga offensive on three fronts
ISIS tightens noose on Mosul residents
Since Mosul's capture by ISIS fighters in June 2014, Mosul has been a vital stronghold for ISIS.
The largest city under ISIS control in Iraq and Syria, it was the city from which the group first declared the establishment of its so-called caliphate.
Since then, ISIS has gradually lost its other Iraqi cities -- Ramadi, Tikrit and Falluja -- to government forces. About 1 million people are estimated to remain in Mosul, once a cosmopolitan trade hub of 2 million residents.