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Bracing for what could be Iraq's bloodiest battle yet

NEAR MOSUL, Iraq -- For more than a year, the Iraqi military and its allies have been talking about and preparing for an operation to try and take back the country's second-largest city from ISIS.

No date has been set for an offensive to retake Mosul, which sits about 250 miles north of Baghdad, on the edge of Iraq's semi-autonomous Kurdistan region.

But Kurdish peshmerga soldiers, who have been the most effective force against the Islamic State of Iraq and Syria in northern Iraq, are getting closer.

Just 20 miles from Mosul, CBS News correspondent Holly Williams watched as peshmerga troops opened fire after spotting what they thought were two ISIS gunmen moving toward their post.

It's no wonder they were nervous; the day before Williams and her CBS News team arrived, there was a coordinated ISIS attack on the peshmerga position.

The Kurdish troops fought the extremists back, and said they had killed almost 100 ISIS militants in the process.

With peshmerga troops, Williams crossed into no-man's land to inspect the aftermath.

"That's ISIS over there... in the village about a mile away," one of the soldiers told her as he gestured at the horizon.

Crumpled pieces of a Humvee, blown up by an ISIS suicide bomber, sit by the side of a road. It's one of the group's most heavily-relied upon tactics, and they'll doubtlessly use many more of the explosives-laden vehicles to defend Mosul.

It's thought that ISIS has several thousand fighters in Mosul.

The extremists have stopped civilians leaving the city, which means, effectively, they have more than 1 million human shields.

When ISIS captured Mosul almost two years ago, many people cheered their convoy as it rolled through the streets.

But Gen. Najim al-Jabouri, the Iraqi army commander leading preparations for the Mosul offensive, told CBS News he's counting on the help of civilians.

"I think about 75 or 80 percent of the people in Mosul, they will support us," al-Jabouri predicted.

But that means about 20 percent of the population in the sprawling city could still back ISIS -- and al-Jabouri knows it.

The general told CBS News he expects the battle for Mosul to last several months.

Even that is optimistic, and the recent battle to retake Ramadi left 80 percent of that smaller city destroyed.