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."

Erdogan says Turkey to take part in Mosul operation
Istanbul (AFP) Oct 17, 2016 –

President Recep Tayyip Erdogan on Monday indicated that Turkey would play a role in the US-backed Iraqi offensive to retake the city of Mosul from jihadists, saying it was unthinkable that Ankara would stay on the sidelines.

"We will be in the operation and we will be at the table," Erdogan said in a televised speech. "Our brothers are there and our relatives are there. It is out of the question that we are not involved."

The build-up for the offensive to retake Mosul from the Islamic State (IS) jihadists who have held Iraq's second city since June 2014 was marked by a bitter squabble between Ankara and Baghdad over the planning.

But Erdogan had indicated that the issue was discussed in talks in the United States over the weekend between Turkish chief of staff General Hulusi Akar and US counterpart Joseph Dunford.

Turkey had raised concern over the possible involvement of Iraqi Shiite and anti-Ankara Kurdish militia in the offensive, while Baghdad had lashed out at the presence of a contingent of Turkish troops in Bashiqa north of Mosul.

But Erdogan said the Turkish troops — whose task is ostensibly to train Sunni anti-IS fighters — were in Bashiqa to stay.

"No-one should expect us to leave Bashiqa. We are there and have made all kinds of operations against Daesh (IS)."

In a sign Ankara was trying to lower the temperature with Baghdad, the state-run Anadolu news agency reported that a top-level Turkish delegation was on its way to the Iraqi capital led by foreign ministry undersecretary Umit Yalcin.