MOSUL, Iraq -- Iraqi government forces backed by the U.S. are on the verge of re-taking the city of Mosul.

But the Islamic State of Iraq and Syria (ISIS) has mounted a bloody last stand in the few neighborhoods it still controls, reports CBS News correspondent Charlie D'Agata.

D'Agata and his team swept into the Old City of Mosul with Iraqi special forces on a cloud of debris, through the twisted maze of streets where die-hard ISIS fighters have dug in. When the roads narrowed too much, they had no choice but to continue on foot.

Miles of the Old City have been reduced to rubble in the fighting CBS News

Cameras can't really do justice to the destruction of the Old City. It just goes on for miles. Whole buildings, whole neighborhoods, just wiped out. This is what it costs to get rid of ISIS.

And yet the fighting continues, with Iraqi forces evacuating their wounded while leaving bodies of ISIS fighters to lie where they fell.

There are still mortars and gunfire flying around in some neighborhoods. D'Agata had to take cover after hearing a round coming, which exploded a few buildings away.

Civilians are still trapped in ISIS-held areas CBS News

No one knows how many residents have weathered the eight months of fighting. Earlier this week, ISIS militants blew up the famous al-Nuri mosque rather than surrender it to Iraqi forces.

It's where the group's leader Abu Bakr al Baghdadi first declared the creation of the so-called caliphate three years ago.

Gen. Abdul Ghani al-Asadi CBS News

"The Russians are now claiming that al Baghdadi is dead. What do you think?" D'Agata asked Iraqi Gen. Abdul Ghani al-Asadi.

"He's not important anymore," al-Asadi said. "As far as we're concerned he's just another ISIS fighter."

For weary soldiers in Mosul, victory means making sure there's nothing left of ISIS.