#MosulOp: Battle against IS 'ahead of schedule'
Baghdad's much-anticipated military operation has met its objectives so far, the Pentagon press secretary Peter Cook said, adding that early indications show the troops are "ahead of schedule for this first day".
But the US official warned the duration of the battle is unspecified, while a top US general said it would take several weeks or even longer.
"We are in the first day of what we assume will be a difficult campaign that could take some time," Cook said.
US backing
A US-led coalition has been assisting and training Iraqi forces involved in the fight for Mosul - the last IS stronghold in Iraq, and providing critical air support.
More than 45,000 Iraqi troops have been trained by the US coalition, while it has launched more than 10,000 "precision strikes" in Iraq - including more than 70 in the Mosul area this month, the Pentagon says.
On Monday, Iraq's prime minister, Haider al-Abadi announced the launch of the military offensive after months of anticipation.
The first day of the offensive left at least 60 IS fighters dead, sources told The New Arab.
The group also lost nearly 60 percent of its rocket launchers and land mines around Mosul in coalition airstrikes and shelling, according to military sources from the Nineveh liberation operations room.
The beginning of the assault also saw aid groups voice fears for the hundreds of thousands of civilians remaining in the city, with IS expected to use them as human shields.
A senior United Nations official on Monday quoted the Iraqi military as saying that large numbers of people may start fleeing fighting in the Mosul area in under a week.
Cook said "there may be as many as seven million leaflets dropped in the next 48 hours or so to try and educate the population of Mosul as to the safest way to conduct themselves as this fighting plays out".
"There is an effort to try and reach out to the people of Mosul to try and make them as aware as possible of what's to come and the dangers they face."
Mosul is the extremist group's last major stronghold in Iraq and its recapture would shatter IS's claim that it is still running a "caliphate" straddling Iraq and Syria.