SHEIKH AMIR, Iraq — The Kurdish pesh merga forces started their advance by moonlight, in the early hours of Monday. East of the Islamic State-held city of Mosul, columns of tanks and trucks lumbered their way toward the objective: clearing villages of militants before any broader advance on the city could happen.
As day broke, the vehicles piled off the roads to avoid any improvised bombs and began moving across the dusty Nineveh plain toward the villages. The tank fire began, booming across the distance. Soon the crawl became a run.
Thick funnels of black smoke began rising from the towns — a past tactic used by the Islamic State militants, setting oil barrels aflame to try to screen them from American airstrikes. The strikes came anyway, sending shock waves through the haze.
The campaign to retake Mosul was officially underway. In the weeks ahead, officials are planning an array of efforts by Iraqi security forces, allied militias, Kurdish forces and air support from the United States as the assault reaches to the city — the most critical population center captured by the militants in their blitz across Iraq in 2014, and now their last major stronghold in the country.
But in the opening hours on Monday, the Kurdish advance on outlying villages east of Mosul was the main action. In response, the Islamic State unleashed at least five suicide vehicle bombs — the militant force’s take on precision-guided weapons.
Near the village of Badana, one vehicle packed with explosives streaked toward the Kurdish positions. A blast and a billowing plume of white smoke proclaimed its fate: Either a ground-fired missile or airstrike had abruptly halted the suicide run, short of its target.
One Kurdish officer, Col. Salar Jabar, put the final toll of the first-day operation at five Kurdish dead and five wounded.
Colonel Jabar, who uses a cane after surviving a car-bomb attack this year, was understated about the challenge. “There was some resistance” was how he put it, insisting that his troops could handle it.
As the Kurds settled into their positions, bulldozers came up to start making walls of sand outside some of the villages — berms against further car bomb attacks.
Kurdish commanders insisted that they were satisfied by the day’s advance, which came in three main columns. Even so, fewer than half of the 10 or so villages they had encircled had been fully cleared, in painstaking and potentially dangerous operations to ensure that no Islamic State fighters were hiding in the settlements.
The monumental challenge that lies ahead was becoming clear. After all, the ultimate objective of the combined assault is not to clear largely depopulated villages, but to reclaim a city of more than a million people that is defended by as many as 4,500 Islamic State fighters. In preparation, the militants have filled trenches with oil, built tunnels and planted copious improvised bombs along the roads to the city and on the streets within it.
“The Iraqi flag will be raised in the middle of Mosul and in each village and corner very soon,” Iraq’s prime minister, Haider al-Abadi, promised early Monday as he announced the start of the assault.
Mr. Abadi has closely tied his political fortunes to the retaking of Mosul, Iraq’s second-largest city, and his announcement of the operation, just before 2 a.m., was hours before the Kurds began their main push.
Under the overall plan to retake Mosul, troops from Iraq’s counterterrorism unit are to link up with the pesh merga in coming days and press forward. That could be the first serious test of Iraq’s premier force in the fight for Mosul.
But political tensions between the Kurds and the Iraqi government are a concern. Toward the end of the day, Massoud Barzani, the president of the Kurdish semiautonomous region, appeared with a senior Iraqi military officer, in a news conference intended to demonstrate that the two sides could work together, at least as far as fighting the Islamic State is concerned.
There is little in the way of shelter or cover along the approaches the pesh merga were taking, and there are many other villages to clear. Smoke from some could be seen from miles away over the flat plain.
The stark nature of the landscape points to another problem ahead: As fighting intensifies, hundreds of thousands of residents are expected to pour out of the city — even though the United States and Iraqi forces are publicly urging the civilians to stay in their homes.
“Basically, we have little less than a week,” Lise Grande, the top United Nations humanitarian coordinator in Iraq, said on Monday. Ms. Grande said that the United Nations had stockpiled supplies — including tents and shelters that can accommodate 400,000 people at the ready — but that there were not yet enough sites ready for temporary camps.
It is also unclear in which direction fleeing civilians will go. The military, Ms. Grande said, has drawn up secret escape routes for civilians that which will be communicated to residents inside Mosul as the battle progresses. This is a lesson learned from the battle of Falluja last summer, when the routes — known as humanitarian corridors — were targeted by the Islamic State after being made public.