NEAR MOSUL, Iraq -- CBS News correspondent Holly Williams is on the new front line in the battle to retake Mosul from ISIS -- a front line that is shifting fast.

Peering over an earthen berm built by Kurdish fighters, Williams could see the forces building a new defensive position even closer to the no-man’s land between them and the entrenched ISIS militants.

The Iraqi Kurds have recaptured about 20 villages in the area east of Mosul from the extremists in the last two days, since the offensive to retake the city began.

Williams is with Kurdish forces along the main road into Mosul. Further down the road is the town of Bartella, which sits just three miles from ISIS territory. It was seized by the Islamic militants when the group blitzed across northern Iraq two years ago.

Bartella was a Christian town, and CBS News visited it just weeks before it was captured by ISIS and spoke to the Christian militiamen who were trying to defend it.

It is -- or was -- an ancient Christian community, but the residents all fled their homes as ISIS encroached. When Williams was there last, she saw residents worshipping in Aramaic, the language that was spoken by Jesus Christ.

If Bartella and the other Christian towns in the area can be recaptured, then the next stop will be the city of Mosul itself.

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ISIS released a propaganda video on Tuesday, attempting to show life going on as normal inside the city. It also showed extremists engaged in firefights, ostensibly trying to defend against the offensive launched by the Iraqi government and its allied Kurdish forces.

But Williams notes ISIS also released images Tuesday of another barbaric execution of a man they claim was a spy. Most of the video is too disturbing to show, but it is typical of the brutality that has also become part of normal life under ISIS and its so-called Islamic State.