BADANA PICHWK, Iraq — Kurdish forces on Monday morning began advancing on a string of villages east of Mosul, the start of a long-awaited campaign to reclaim Iraq’s second-largest city from the Islamic State, which seized it more than two years ago, officials said.
About 4,000 Kurdish pesh merga troops are involved in the operation to retake 10 villages, the opening phase of a battle that could take weeks or months and could involve nearly 30,000 Iraqi and Kurdish troops, with American warplanes providing air support. Iraqi counterterrorism forces, which work closely with American Special Operations commandos in Iraq, are also expected to join the Kurdish forces in the coming days.
The operation began hours after Prime Minister Haider al-Abadi announced in a brief speech aired on state television just before 2 a.m. that the long-awaited campaign to liberate Mosul had begun.
“The Iraqi flag will be raised in the middle of Mosul, and in each village and corner very soon,” Mr. Abadi said, dressed in a military uniform and surrounded by officers.
Defense Secretary Ashton B. Carter, in Washington, said the start of the Mosul campaign was a “decisive moment” in the effort to defeat the Islamic State.
In the first phase, the troops who have been massing at bases around Mosul in recent weeks will encircle the city, seeking to cut it off and prevent Islamic State fighters from fleeing, particularly west into Syria. Later, the counterterrorism forces, which took the lead in liberating other Iraqi cities, like Ramadi and Falluja, from the Islamic State, will join regular army units in storming the city.
After dark on Sunday evening, armored vehicles on flatbed trucks were seen moving west from Erbil, the capital of the autonomous Kurdish region, along with many ambulances.
The operation began under the light of the moon on Monday as pesh merga tanks, Humvees and pickup trucks with guns mounted on the back snaked their way toward the villages.
Although Mr. Abadi vowed that the Iraqi flag would be flown in every town, only the tricolor flag of the Kurdish semiautonomous region could be seen during the assault.
The Kurdish troops included the elite Zeravani paramilitary force, which attacked on three fronts. To avoid roadside bombs, a pesh merga column drove off the main highway, headed south on a rutted, undulating dirt road, and it then rumbled west across a dusty field.
There has been considerable speculation about how hard Islamic State fighters would resist: Would the militants make a final stand in the villages or pull back to Mosul to fight another day? The sounds of battle Monday morning indicated there was resistance.
Attack helicopters could be heard overhead at the start of the assault, followed by the thud of tank rounds as the pesh merga fired on Islamic State positions across a stretch of the Nineveh Plain. There were bursts of machine-gun fire. A powerful airstrike sent out shock waves. In the distance, there was a funnel of black smoke.
Pesh merga officers said they expected to be joined in a couple of days by Iraqi forces, which would help them secure their gains and, ultimately, push farther west. But first there were villages to take and secure, and the goal for the pesh merga on Monday was to take control of more than 45 square miles.
The battle plans have unfolded in recent weeks against a backdrop of grave concerns about the civilian population in Mosul, which by some estimates numbers more than one million. United Nations officials have warned that 200,000 people could be displaced in just the first few days of an assault on the city, and humanitarian groups have scrambled to set up emergency camps for the displaced.
Hoping to avert a mass exodus of civilians, which could place them in the middle of the crossfire, the Iraqi government, in radio broadcasts into Mosul and with thousands of leaflets dropped over the city, has urged civilians to stay in their homes.
The leaflets gave a long list of instructions: Put tape over windows in the form of an X to prevent shattering. Disconnect gas pipes. Hide jewelry and money. Stay on low floors. Tell your children that the loud booms are just thunder.
And for the young men of Mosul, the government had a special instruction: Rise up against the Islamic State when the battle begins.
The campaign had been building for months, and Mr. Abadi’s dramatic late-night speech was in keeping with tradition. Earlier this year, he made similar remarks late at night before an offensive on Falluja, which was retaken in June after several weeks of fighting.
Addressing the people of Mosul and using an Arabic acronym for the Islamic State, Mr. Abadi said, “Today I declare the start of these victorious operations to free you from the violence and terrorism of Daesh.”
Mosul, a Sunni-majority city that once had a diverse population that included many Shiites and Christians, fell to the Sunni jihadists of the Islamic State in June 2014, when soldiers of the Iraqi Army, built up with tens of billions of dollars of support from the United States, dropped their weapons, shed their uniforms and ran. Initially, many Sunni residents of Mosul, angered at what they perceived as second-class treatment from the Shiite-led government in Baghdad, welcomed the militants.
But after more than two years of the terrorist group’s brutal rule, with public beheadings and tough rules that ban smoking and force women to cover themselves in public, many residents have grown tired of the jihadists.
It was in Mosul, from the pulpit of the city’s Great Mosque, that Abu Bakr al-Baghdadi, the leader of the Islamic State, declared a caliphate, or Islamic state, that spanned the borders of Iraq and Syria.
By MICHAEL R. GORDON and TIM ARANGO
Michael R. Gordon reported from Badana Pichwk, and Tim Arango from Erbil, Iraq. Matthew Rosenberg contributed reporting from Washington.