The battle for the ISIS-held city of Mosul, now in its second day, is expected to drag on for weeks or months. As Kurdish and Iraqi forces approach the city, aid groups in the region are preparing for a humanitarian crisis.

Fighting has lulled in some areas, but is continuing in others, and airstrikes by the U.S.-led coalition continue, NPR's Alice Fordham reports from Kalak, Iraq.

"The Iraqi army is fighting its way toward the city from the south: a spokesman said they are facing resistance but moving," Alice says.

"To the east of Mosul, Kurdish Iraqi forces say they have stopped fighting for the moment but are moving artillery to a northern front for an offensive that could begin later today or tomorrow."

Meanwhile, the International Committee of the Red Cross, which is calling for all sides to spare civilians, says it's prepared to assist 270,000 people.

The group's regional director tells Reuters that ICRC medical centers are equipped to treat any civilians affected by chemical weapons.

Mosul was once home to 2 million people, and it's believed that up to a million might still be in the city.

Retired Army Col. Peter Mansoor, who served twice in Iraq, tells NPR that while Kurdish and Iraqi forces outnumber ISIS fighters "by more than 10 to 1" — and have the backing of the U.S.-led coalition's firepower — the battle will be a slow and bloody grind.

"Urban combat is never fast and it's never easy," he said.

He notes that a corridor has been left open to allow civilians to leave the city and move west, but "it's unlikely that ISIS is going to allow the population to flee," he says.

"ISIS, I think, probably wants a humanitarian catastrophe in Mosul. It's part of their narrative that this is the West versus Islam — 'and see what happens when they support forces that attack us,' " he says. "So the population is likely to suffer a great deal, unfortunately."

As NPR reported yesterday, the battle is a crucial one, both strategically and symbolically:

"The battle could be a watershed moment in the fight against the Islamic State. Mosul — one of Iraq's largest cities — is the last major urban center in the country under ISIS control. ..."It's expected to be a difficult fight for Iraq's security forces, which suffered a rapid and humiliating defeat there two years ago. Mosul is much larger than other recently liberated cities such as Ramadi and Fallujah."

Mansoor notes that the military challenges include roadside bombs, thousands of fighters ready to fight to the death and burning pools of oil blocking visibility.

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