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Forces closing in on Mosul may face ISIS suicide bombers and tunnel network

TARJELA, Iraq -- The village of Tarjela was recaptured from ISIS Monday, and in the rubble left behind by the battle are clues for how ISIS will defend Mosul.

The extremists dug a network of tunnels here to protect themselves from attack.

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The entry to an ISIS tunnel

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Lt. Col. Dilshad Salim said he expects the same tactics in Mosul, as well as more suicide bombers.

The battle for Mosul began in villages on the eastern side of the city, where Kurdish fighters went house to house on Monday hunting down the ISIS gunmen holding out.

U.S. airstrikes demolished the tunnels in Tarjela, which was abandoned by its residents over two years ago.

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Aftermath of an airstrike

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But in Mosul, home to around one million civilians who are effectively human shields, airstrikes will be difficult without massive casualties.

ISIS released propaganda video Tuesday claiming that life in the city is normal, and showing the extremists fighting off the offensive.

But normal life in Mosul also includes a warning; Another brutal execution of a man they claim is a spy.

ISIS is hunkering down, preparing to defend the biggest city in its so-called Islamic state.

There are also fears that ISIS could use chemical weapons during the battle for Mosul. In September, American airstrikes destroyed what the U.S. says was a chemical weapons plant in the area.