MOSUL, Iraq (AP) " Hundreds of Iraqi civilians continued to flee Mosul on foot Thursday as the Islamic State group launched fierce counterattacks on Iraqi positions along the city's southwestern edge. The fight to rout IS from western Mosul, the last urban IS bastion in Iraq, is causing higher numbers of displaced people than previously seen in the 4 ½-month-long offensive for Iraq's second-largest city, according to a statement from the United Nations on Wednesday. The U.N. said that 28,400 people have been forced from their homes since the operation began more than a week ago. Inside Mosul's Mamun neighborhood IS fighters trapped deeper inside the city launched salvos of mortar rounds, targeting Iraqi positions and along routes used by civilians to flee. "There are so many families in Mosul, it turns all of our missions into two missions: protect the civilians and secure our own location," said Iraqi special forces Lt. Nour Sabah. Sabah's men hold an old school building in Mamun from which they can look down into IS positions just a few blocks away. Despite holding the area for more than three days, Sabah said IS launched an RPG attack on his forces on Thursday morning that wounded one of his men. The push to free western Mosul began on February 19. Iraqi forces first retook Mosul airport and the sprawling Ghazlani military base next to it, but are still struggling to secure a strip of the city's southwestern neighborhoods where dense clusters of houses and thousands of civilians are frustrating the fight. Mosul's eastern section " the city is divided by the Tigris River into a western and an eastern half " was declared "fully liberated" in January, after weeks of grueling urban combat. The operation to retake Mosul from the Islamic State group was formally launched in October. The offensive came after the militants were slowly pushed out of most of the territory they overran in the summer of 2014. At the height of the group's power, IS controlled nearly a third of Iraq. Western Mosul is now the last significant pocket of urban territory the militant group holds in Iraq.