BAGHDAD (AP) — Iraqi special forces on Thursday joined the government offensive to rout Islamic State militants from the western half of Mosul, pushing up to a sprawling, IS-held military base on the city's southern edge as police forces advanced toward the adjacent international airport, officials said.

From the south, the elite counterterrorism forces reached the Ghazlani base in the morning hours, two special forces officers said, adding that fierce clashes are underway there at the base perimiter.

A federal police officer told The Associated Press that his forces were pushing toward the Mosul airport. All officers spoke on condition of anonymity because they were not authorized to talk to reporters.

The advances come days after Iraqi forces officially launched the operation to take Mosul's western half, with the Iraqi regular army and federal police forces taking part in the initial push. Since the operation began on Sunday, Iraqi forces say they have retaken some 120 square kilometers — nearly 50 square miles — south of the city.

"The counterterrorism forces will be an additional force, which will expedite the liberation of Mosul's western side," Brig. Gen. Yahya Rasool, an Iraqi military spokesman, told the AP.

The battle for western Mosul, the extremist group's last major urban bastion in Iraq, is expected to be most daunting yet.

The streets are older and narrower in this section of the city, stretching west from the Tigris River that divides Mosul into the eastern and western half. The dense urban environment will likely force Iraqi soldiers to leave the relative safety of their armored vehicles. The presence of up to 750,000 civilians will also pose a challenge.

Mosul fell to IS in the summer of 2014, along with large swaths of northern and western Iraq.