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The addition of the elite troops, also known as counterterrorism forces, marked a significant intensification of the fight for Iraq's second-largest city.

As they advanced, attack helicopters fired on the militants and heavy gunfire echoed across the plains.

Major General Maan al-Saadi said the elite Counterterrorism Forces advanced on the town of Bartella with the aid of US-led coalition airstrikes and heavy artillery on the fourth day of a massive operation to retake Iraq's second-largest city.

"God willing, we will take this town today," he said.

The militants fought back, unleashing at least four suicide car bombs against the advancing forces, one of which blew up after it was struck by tank fire.

It was not immediately clear if there were any casualties from the fighting.

The special forces are expected to lead the way into Mosul, where they will face fierce resistance in an urban landscape where IS militants are preparing for a climactic battle.

Peshmerga soldiers walk through the rubble at  Bartella village after it was hit by a coalition airstrike [Getty]

The Kurdish forces known as Peshmerga, who are also taking part in the offensive, announced a "large-scale operation" to the north and northeast of Mosul on Thursday.The offensive is the largest operation launched by Iraqi forces since the 2003 US-led invasion, and is expected to take weeks, if not months.

"The operation will be in three fronts," the Peshmerga said in a statement, and follows recent gains by the Peshmerga to the east of Mosul and Iraqi security forces to the south.

Peshmerga forces stationed on mountains northeast of Mosul descended from their positions and charged toward the front line.

They used bulldozers and other heavy equipment to fill trenches and moved armoured vehicles into the breach after about an hour of mortar and gunfire at IS positions below in the village of Barima.

Military operations also appeared to be underway in the town of Bashiqa, northeast of Mosul. Thick smoke could be seen billowing from the town early on Thursday.

Thick smoke could be seen billowing from the town early on Thursday

A day earlier, Bashiqa was pounded by airstrikes and mortar fire from Kurdish Peshmerga positions high above.

The approaches to Mosul run through clusters of mostly abandoned villages where IS militants have planted roadside bombs and other booby traps.

Bartella, a traditionally Christian town which fell to IS two years ago, is believed to be empty of civilians.

"Our intelligence tells us the district is full of IEDs," al-Saadi said, referring to the homemade explosives IS has planted in huge numbers during past campaigns.

Amer al-Jabbar, a 30-year-old soldier with the Iraqi special forces, said he was happy to be taking part in the attack and hoped to avenge two brothers killed while fighting for the Iraqi security forces.

"I had one brother who became a martyr in 2007 and another who became a martyr in 2014," he said. "I want to avenge them and I'm ready to die."

Another soldier called in coordinates for airstrikes by Iraqi attack helicopters hovering overhead. One of the helicopters was struck by a sniper, but managed to land safely.

An intense gun battle erupted as the convoy of some 1,000 special forces made its way toward Bartella. A road sign said the Mosul city centre was 27 kilometres [17 miles] away.

IS captured Mosul during a lightning advance across northern Iraq in 2014, and IS leader Abu Bakr al-Baghdadi announced the formation of a self-styled caliphate from the pulpit of a Mosul mosque.

More than 25,000 forces, including the Iraqi army, the peshmerga, Sunni tribal fighters and Shia militias are taking part in the Mosul offensive, which began Monday after months of preparation. They will be advancing on the city from several directions.

The special forces advanced in some 150 Humvees decked with Iraqi flags and Shia religious banners. Ali Saad, a 26-year-old soldier, said the Kurdish forces had asked them to take down the religious banners, but they refused.

"They asked if we were militias. We said we're not militias, we are Iraqi forces, and these are our beliefs," he told AP.

Mosul is a Sunni majority town, and many fear the involvement of the Shia militias in the operation could stoke sectarian tensions.

The Shia militias have said they will not enter the city itself, but will focus on retaking the town of Tel Afar to the west, which had a Shia majority before it was captured by IS.