Updated

Iraqi special forces joined the Mosul offensive on Thursday with a pre-dawn advance on a nearby town held by the Islamic State group, encountering heavy fire.

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

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.

The offensive is the largest operation launched by Iraqi forces since the 2003 U.S.-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 northeast of Mosul on Thursday.

"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.

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 15 kilometers (nine miles) from Mosul.