Iraqi forces `ahead of schedule` in Mosul: Pentagon

District of Columbia: Iraqi security forces were "ahead of schedule" after the first day of an offensive for the Islamic State-held city of Mosul, a Pentagon official said Monday.

A US-led coalition has for months been helping train Iraqi forces for the fight for Mosul -- the last IS stronghold in Iraq -- and the military offensive finally got underway early Monday.

"Early indications are that Iraqi forces have met their objectives so far, and that they are ahead of schedule for this first day," Pentagon press secretary Peter Cook said.

But he warned it was unknown how long the battle would last. A top US general earlier 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.

After IS overran large parts of Iraq and Syria in early 2014, the United States formed a coalition that launched an air campaign to strike the Sunni extremists and to train local, partnered forces to do the fighting.

The coalition has trained more than 45,000 Iraqi troops and launched more than 10,000 precision strikes in Iraq -- including more than 70 in the Mosul area this month, the Pentagon says.

The start of the long-awaited assault has raised deep concerns for hundreds of thousands of civilians trapped in Iraq`s second-largest city, with aid groups warning of a massive humanitarian crisis.

The Iraqi government has dropped thousands of leaflets on Mosul telling residents what to do during the offensive.

"My understanding is that 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," Cook said.

"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."