ISIS group holding civilians in Iraq's Mosul: Pentagon

District of Columbia: Islamic State jihadists were barring civilians from leaving Mosul on Tuesday and using them as human shields, as the battle for the Iraqi city entered its second day, a Pentagon official said.

"We know they are being used as human shields, absolutely," Pentagon spokesman Navy Captain Jeff Davis told reporters.

"They are being held there against their will. We have not seen any change in the last day of people leaving or fleeing."

Authorities and aid agencies are bracing for a massive flow of civilians fleeing Mosul as the fight progresses. 

Currently, the action has largely been in villages surrounding Iraq`s second-largest city and the remaining IS stronghold in the country.

President Barack Obama warned of "significant" displacement but said the United Nations and other aid groups were ready.

"We have put together plans and infrastructure for dealing with a potential humanitarian crisis that are as extensive as the military plans," Obama told reporters earlier.

United Nations spokesman Stephane Dujarric said an estimated 200,000 people are expected to be displaced in the first weeks of the offensive, growing to as many as one million under a worst-case scenario.

But "what you are not seeing is a mass exodus of civilians, and that`s because they are being forcibly held there," Davis said.

The fighting so far has been fairly light, mainly focused on traditionally Kurdish villages east of Mosul that Kurdish peshmerga fighters are reclaiming from IS.

Davis said jihadists were deploying suicide-car attackers and had been attempting to thwart air raids and intelligence drones by igniting giant pits full of oil and tires to create thick smoke clouds to hide their movements and positions.

"We`ve seen very good progress," he said. 
"It`s going to be a while. There`s a lot of movement they have to do to get in (to Mosul) but it`s very much under way."