General Maan al-Saadi said the elite Counter-terrorism Forces advanced on the town of Bartalla with the aid of US-led coalition air strikes and heavy artillery on Thursday, the fourth day of a massive operation to retake Iraq's second-largest city.

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

The offensive is the largest operation launched by Iraqi forces since the 2003 US-led invasion. It is expected to take weeks, if not months.

The Kurdish forces known as peshmerga, who are also taking part in the offensive, announced a "large-scale operation" to the north and north east of Mosul on Thursday.

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

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."

An intense gun battle erupted as the convoy of some 1,000 special forces made its way toward Bartalla. The troops were around nine miles from Mosul.

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.

Mosul is the largest city controlled by the extremist group and its last major urban bastion in Iraq.

Iraq's US-trained special forces are seen as far more capable than the security forces that crumbled as IS advanced in 2014. They have played a central role in liberating several cities and towns over the past year, including Ramadi and Fallujah, in the western Anbar province.

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

The US military is carrying out air strikes and artillery shelling in support of the operation. More than 100 US forces are embedded with the Iraqis, and hundreds more are playing a supporting role in staging bases.

Maj Gen Gary Volesky, the top commander of US land forces in Iraq, said army Apache attack helicopters were striking IS targets in support of the operation.