JUDY WOODRUFF: Now: Mosul and the start of operations to retake Iraq's second largest city from the Islamic State group.

Jeffrey Brown will speak with a former U.S. ambassador to Iraq in a moment, but, first, chief foreign affairs correspondent Margaret Warner reports on today's events.

MARGARET WARNER: Gunfire sounded all day, across the outskirts of Mosul. Columns of smoke billowed from artillery fire and U.S. coalition airstrikes and from oil ignited by ISIS fighters to blind attacking planes.

Long anti-ISIS convoys advanced, and, by late in the day, the overall Iraqi commander issued a confident assessment.

LT. GEN. TALIB SHAGHATI, Iraqi Army (through translator): The operations are going very well and according to plan. Sometimes, we are ahead of the plan because of the high morale and the fighters' strong will to fight the Islamic State group and liberate Mosul, which has been under ISIS rule for the past two years.

MARGARET WARNER: For the Iraqi army, the campaign for Mosul is by far its largest operation yet against the Islamic State. U.S. intelligence estimates that up to 4,500 ISIS fighters are in the city. Other estimates run to 8,000.

LT. GEN. STEPHEN TOWNSEND, U.S. Army: This may prove to be a long and tough battle, but the Iraqis have prepared for it, and we will stand by them.

MARGARET WARNER: Complicating matters, balancing the various factions taking part in the fight. Kurdish forces began today by seizing several towns on the eastern fringes of Mosul. They will be part of a five-pronged assault, including Iraqi army brigades, plus Sunni tribal fighters and police. The ultimate drive into Mosul will likely be made by Iraqi special forces.

Providing air cover and assisting on the ground is the American military, including special operations troops. Shiite militias backed by Iran are also taking part, but they have been accused of atrocities against Sunnis in other cities and may be kept out of Mosul proper.

Meanwhile, Turkey's president, Recep Tayyip Erdogan, complained of being left out of the Mosul offensive.

PRESIDENT RECEP TAYYIP ERDOGAN, Turkey (through translator): Look now. The operation in Mosul has started. The operation continues. And what do they say? Turkey shouldn't enter Mosul. How can I do that? I share 350 kilometers of border with Iraq, and I am under threat on that border.

MARGARET WARNER: Erdogan said again he will keep some 3,000 Turkish-trained fighters in northernmost Iraq, despite objections from Baghdad. As the fighting starts in earnest, concerns are also rising over the more than one million civilians still in the city. The U.N. and other organizations say they're preparing for up to 200,000 refugees.

For the "PBS NewsHour," I'm Margaret Warner.