The complex military battle to retake Mosul from the Islamic State has begun. Here is what you need to know about the ancient Iraqi city. (Ishaan Tharoor, Kareem Fahim, Jason Aldag/The Washington Post)

Iraq’s elite counterterrorism units advanced to within six miles of Mosul on Thursday as Kurdish forces opened a new front to the north — in a significant escalation of the fight for the Islamic State-held city.

Plumes of dust and smoke rose over the majority-Christian town of Bartella, east of Mosul, as Islamic State militants sent a barrage of car bombs to repel the advance of the counterterrorism forces. But by nightfall, the militants’ resistance had crumbled and the Iraqi flag had been raised over the town’s main church, commanders said.

Kurdish forces said they had launched a “large-scale” offensive from the north to secure a string of villages.

The multipronged attacks raised the tempo of the battle for the city, which began Monday. The entry onto the battlefield of the black-clad elite forces — who have trained closely with the U.S. military — marks the beginning of what their commanders say will be a sharp push toward Mosul, the militants’ last major stronghold.

Iraqi Prime Minister Haider al-Abadi announced to Western diplomats in a video call to Paris that forces were moving “more quickly than planned” toward Mosul, and he lauded the cooperation
in the attack by the different ­forces.

Iraqi counterterrorism commanders said they had advanced a total of eight miles in their push toward the city. On the northern front, Kurdish forces moved into the village of Nawaran.

Despite the advances, the militants showed that they were not willing to concede ground easily. In the north, a U.S. service member was killed by the explosion of an improvised explosive device, the Pentagon said.

There were indications that there were also heavy casualties among Kurdish peshmerga forces, which Kurdish forces linked to a lack of air support from the U.S.-led coalition.

“Regrettably a number of peshmerga have paid the ultimate sacrifice for us to deliver today’s gains,” the peshmerga’s general command said in a statement. Coalition support and air cover “were not as decisive as in the past,” it added. The Islamic State claimed five suicide attacks on the peshmerga, killing an unspecified number of Kurdish soldiers.

In Bartella, to the east, once home to 30,000 people, Iraqi units were backed by attack helicopters as they broke through the earthen berms that once marked the forward line. One helicopter was hit by Islamic State artillery fire, although the pilot managed to land and crew members were evacuated, said Gen. Hamid al-Maliki, head of army aviation.

The Islamic State militants had dispatched an onslaught of 15 car bombs, and while most were eliminated by tank fire or rockets, at least one hit its target.

“They are trying to shock us and break our morale,” Lt. Gen. Abdulwahab al-Saedi, a counterterrorism commander, said from an observation point atop a deserted strip mall on the edge of the town as another truck bomb exploded in the distance.

The elite units have been at the front of nearly every other battle against the militants in Iraq as territory has slowly been taken back from the Islamic State. The units expect to be the spearhead once more in Mosul and have about 3,000 troops involved in the operation.

The battle for Mosul is by far the most complex fight in the grinding conflict to push Islamic State militants out of Iraq, involving not just regular Iraqi army forces but also Sunni tribal units, powerful Shiite militias and the Kurdish troops of the northern semiautonomous region — forces that are often at odds with the Baghdad government.

“We’re the only ones with the capability,” Saedi said.

Mosul has been under Islamic State control for more than two years and is the group’s last remaining major urban center in Iraq. Losing it would drive the Islamic State back to being the insurgency it once was, before it began seizing territory in the country in 2014.

While it may not be far to the outer limits of the city, Saedi said he expects the work on the outskirts to be the toughest part of the fight. Inside the city, the presence of at least 1 million civilians prevents the Islamic State from heavily booby-trapping areas.

“They always focus on their outer lines,” said Saedi, who also commanded offensives in Tikrit and Fallujah.

Heavy exchanges of gunfire and explosions could be heard for most of the day as the wounded were rushed away from the front in ambulances and armored vehicles. At a field clinic behind the front lines, Sgt. Maj. Haider Salal, who mans the machine gun on the turret of a Humvee, was recovering from a gunshot after sniper fire entered the vehicle.

“The shooting was very intense when we opened up their defense line,” he said. It was the third time he had been injured in his six years with the Iraqi counterterrorism forces. “This one was a hard fight,” he said.

In the bed next to him, Hamza Abdel Kareem, a noncommissioned officer who is also a gunner, was suffering from shock after a rocket-propelled grenade hit the side of his vehicle. He said the militant who fired it appeared from a tunnel.

“He just came out of the hole. I didn’t see him fast enough,” Kareem said. The Humvee was hit, but no one was seriously injured, he said. Within hours, he asked to go back to the fight.

Those who were more seriously hurt in the fighting were rushed to hospitals in the Kurdish capital of Irbil. Commanders declined to release the number of casualties in the operation but said 80 Islamic State fighters were killed.

Intense fighting continued well into the afternoon, and it was unclear how tight a grip Iraqi forces had on the village by the end of the day, although commanders said it was under full control.

“Bartella is the eastern gate to Mosul,” said Maj. Gen. Maan al-Saadi, with the counterterrorism units. “Now the road is open.”

Fahim reported from Irbil. Mustafa Salim in Irbil and Aaso Ameen Shwan in Bartella contributed to this report.