MOUNT SINJAR, Iraq — At 7 a.m., the Kurdish forces began their assault.
Backed by a barrage of U.S.-led airstrikes, about 7,500 Kurdish peshmerga fighters, including thousands of minority Yazidis, launched a three-pronged attack against the Islamic State in the northern Iraqi city of Sinjar.
They had gathered at the base of Mount Sinjar at dawn Thursday. Some prayed and others huddled by campfires before moving to their positions. The dirt tracks to the front lines were jammed with vehicles full of fighters.
On the western side of the mountain, Kurdish special forces tentatively advanced on foot into Islamic State-held territory followed by a snaking convoy of armored vehicles. By nightfall they had succeeded in cutting the highway next to the city, which runs from Raqqa in Syria to Mosul in Iraq, splitting the Islamic State’s territory apart.
Peshmerga forces had entered a former Iraqi military base and cleared a string of villages, Kurdish officials said.
The drive to retake Sinjar is the largest offensive launched by Iraqi Kurds against the Islamic State and a key test of their military capabilities. It comes as the militants face attacks on multiple fronts, from Raqqa, the group’s de facto capital in Syria, to Ramadi in western Iraq.
The loss of Sinjar would deal the Islamic State a severe setback, cutting its supply lines between Iraq and Syria. But it is also a particularly emotive fight.
The sudden fall of Sinjar to Islamic State militants in August 2014 devastated the Yazidi community. Hundreds of thousands fled Sinjar and the surrounding area — many straight into the hands of the extremists, who consider Yazidis heretics. Yazidi men were summarily executed en masse, and women were rounded up to be bought and sold as sex slaves. Tens of thousands of people fled to Mount Sinjar.
As troops gathered Thursday at dawn at the base of the mountain to launch their attack, the Yazidi fighters pledged vengeance.
“It was a tragedy, and we carry a great sorrow in each of us,” Salim Shevan, a 28-year-old Yazidi fighter, said as he left for the front lines. “We will have revenge.”
After the Kurdish forces began advancing, bulldozers broke through the earthen barrier that previously marked the front line to make way for dozens of armored vehicles, led by a U.S.-
supplied MRAP, designed to withstand roadside bombs.
While the closest villages appeared abandoned, the attackers soon encountered resistance as they turned toward Sinjar.
In the distance, a vehicle sped toward the convoy from the direction of Syria.
“Suicide bomber! Suicide bomber!” Brig. Gen. Rawan Barzani, the special forces commander and son of the Kurdish region’s president, radioed to his men on the front line. The convoy fired two antitank missiles, but they missed. Another finally hit as the car neared the convoy, and a plume of gray smoke rose into the air.
Sinjar lies on Highway 47, the route used by the Islamic State to transport fighters, weapons and commodities such as oil.
“Getting Sinjar is crucial because then [the Islamic State] has to decide between Raqqa and Mosul,” the group’s stronghold in northern Iraq, Barzani said. “It won’t be able to hold both.”
Kurdish commanders said they expected the offensive to last about a week. Despite quick progress made Thursday, few believed the fighting in the city will be easy.
Pentagon press secretary Peter Cook said that advisers from the United States and other countries were taking part in the Sinjar operation but declined to provide the number of foreign forces involved.
“Most of those folks . . . are behind the front lines, advising and working directly with peshmerga commanders,” Cook said. “There are some advisers who are on Sinjar Mountain, assisting in the selection of airstrike targets.”
British officials said they are also supporting the operation. “The Royal Air Force has been playing a full part in coalition reconnaissance and strike missions to provide effective air support to them and other Iraqi ground forces,” a Defense Ministry spokesman said.
U.S. officials estimate that about 400 to 550 Islamic State militants are in the city.
Lt. Col. Dilgash Zebari, a peshmerga commander, said militants have dug tunnels and hideaways in the city. “We’ve had suicide bombers; we are expecting more when we go inside,” he said.
If the offensive bogs down, rivalries between various factions fighting the Islamic State will be to blame, Zebari said.
The long-planned operation had stalled for weeks because of bad weather and political wrangling between factions of Kurdish soldiers. Fighters affiliated with the Turkish-based Kurdistan Workers’ Party, or PKK, have held positions in the city, but Kurdish peshmerga forces launching the offensive said they are not directly coordinating with them.
“If the liberation takes too long, it will be because of having troops from different parties, each of them flying their own flag,” said Zebari.
The Kurdish-led efforts also have significance in the wider efforts to cripple the Islamic State. In Syria, Kurdish militiamen emerged as a linchpin in Pentagon plans after the failure of Washington’s strategy to arm and train other Syrian rebel units to fight the Islamic State.
Syrian Kurds have waged a series of strikes against the Islamic State and last year withstood the group’s siege of Kobane, a town near the Turkish border.
But the rising profile of Kurdish fighters poses potential complications for the U.S.-led alliance. Turkey, a NATO member that has battled PKK separatists for decades, has deep concerns over growing Kurdish political and military influence, fearing it could encourage greater calls for autonomy.
The U.S. military said Thursday that it dispatched an additional six F-15E fighter jets to the Incirlik air base in southern Turkey to join the air campaign against the Islamic State.
Bryan Murphy, Missy Ryan and Karen DeYoung in Washington contributed to this report.