Iraqi forces have been using bulldozers to clear front lines in Mosul. (Reuters)

Iraqi forces faced snipers, mortar fire and booby traps as they began an assault on Mosul’s Old City on Sunday, breaking into a maze of narrow streets and alleyways where hundreds of hardened Islamic State militants are expected to make a bloody last stand.

Clouds of smoke rose above the historic city center in the early morning as a barrage of artillery and airstrikes from U.S.-led coalition jets struck militant targets.

From nearby buildings bulldozers could be seen attempting to break through the barricades that marked Islamic State defense lines, coming under heavy fire but eventually opening the way for counterterrorism forces that led the assault.

Intense gun battles broke out after they entered. With the winding streets making car bombs more difficult to mobilize, the militants compensated with antitank weapons and mortar fire, which crashed down into streets behind the front lines. Snipers also waylaid the advance.

“They are besieged, they will fight to the death,” said Master Sgt. Latif Omran, as his unit, armed with M-4 assault rifles and rocket-propelled grenades, waited just back from the front line for Humvees to ferry them forward.

Over the past eight months, the militants have been gradually corralled into the Old City — an area of little more than a square mile on the western banks of the Tigris River.

The loss of their last foothold in Mosul, once the largest city the militants controlled, will strike a huge symbolic blow to the Islamic State. It was in the Old City’s Great Mosque of al-Nuri that the group’s leader, Abu Bakr al-Baghdadi, declared the formation of a caliphate three years ago.

Since then the group has lost the majority of its territory in Iraq, while an offensive for Raqqa, the Islamic State’s Syrian capital, began last month.

However, despite the losses few expect an easy fight for the last few inches of Mosul, where the United Nations estimates that as many as 150,000 civilians remain trapped.

The tiny lanes of the Old City make the terrain particularly challenging for Iraqi forces, as they can’t enter many areas with their armored vehicles. Much of the fighting will have to be done on foot.

“This is their forward defense line so there’s fierce resistance,” said Lt. Gen. Abdelwahab al-Saedi, deputy head of the counterterrorism forces, at a base in western Mosul. “They are using the mortar shells heavily.”

As the assault began, so did the inevitable casualties. Minutes after Omran and his unit left for the front came a crackle over the radio. “Our gunner is injured — we need another,” came the voice of one officer.

While the counterterrorism forces lead the assault into the center of the Old City, moving east toward the river, army and police forces are supporting their flanks.

On the other side of the front lines, terrified families are trapped in their houses. Sheltering in crowded basements, many have not seen sunlight for weeks. Humanitarian agency workers have urged Iraqi and coalition forces to use caution and restrain the use of heavy weaponry.

“The buildings of the old town are particularly vulnerable to collapse even if they aren’t directly targeted, which could lead to even more civilian deaths than the hundreds killed so far in airstrikes across the rest of the city,” said Nora Love, the International Rescue Committee’s acting country director.

Iraqi commanders and the U.S.-led coalition say they are taking into account the integrity of the buildings and the fact that the militants are using civilians as shields as they carry out strikes. Still, civilians who have managed to dodge Islamic State snipers to flee, and those still trapped inside, say civilians are dying every day in the bombardment.

As the assault began in the morning, three TOS-1 missiles sailed into the city. The thermobaric rockets cause a blast of pressure and can kill over an area of 3,000 square feet in open terrain. They were used on school buildings known to be devoid of civilians, said Col. Arkan Fadhil, who coordinates airstrikes with the coalition. The pressure blast can be contained by surrounding buildings in urban areas, he said.

Counterterrorism forces had taken two of the school buildings by the end of the day, giving them a “foothold” in the Old City, he said.

About 75 Islamic State militants manned the forward defense line — in groups of two or three — he said. Saedi said counterterrorism forces had taken about 150 yards by the early afternoon. Federal police forces also claimed to have gained around 150 yards.

Instead of being told to stay in their homes as they have been elsewhere, civilians will be asked to evacuate both for their own protection and to make the neighborhood easier to clear, according to commanders.

Over the past five days, loudspeakers have blared into the Old City, promising “salvation” and urging Islamic State forces — whose numbers are difficult to estimate — to surrender.

But as many as 400 of those inside are hardened foreign fighters, who are likely to fight to the death, said Lt. Gen. Sami al-Aridhi, also a counterterrorism commander. Some Iraqis fighting for the Islamic State have managed to flee with displaced families, he said.

While there is not much ground to cover, some commanders predict the final push for Mosul could last at least a month.

The elite counterterrorism units — which have led the majority of Iraq’s fight against the militants over the past three years — have suffered a 40 percent casualty rate since the beginning of the operation, according to U.S. figures, raising concerns about how long they can sustain a prolonged battle. Saedi said the casualty rate remained in the “acceptable range.”