BAGHDAD (AP) — Iraqi federal police pushed their way into the perimeter of Mosul International Airport on Thursday, taking control of the runway amid fierce exchanges of fire with Islamic State militants hunkered down in several airport buildings, police officials said.

The advance came as part of a major assault that started five days earlier to drive the Islamic State group from the western half of Mosul, Iraq's second-largest city.

Two police official told The Associated Press that after police forces pushed onto the airport grounds and seized the runway, they came under heavy fire from inside the buildings at the site, including the main airport building.

The officials would not provide more details but said heavy exchanges were underway. They said troops from the U.S.-led coalition were with the advancing forces, though they didn't specify the nationalities of the foreign forces. The officials spoke on condition of anonymity because they were not authorized to talk to reporters.

Private broadcaster Lebanon-based Al-Mayadeen aired live footage from the Mosul airport perimeter, showing a military helicopter buzzing overheard and firing at IS positions as gunfire rattled.

Earlier on Thursday, Iraqi special forces joined the government offensive for the western half of Mosul, pushing up to a sprawling, IS-held military base on the city's southern edge that's adjacent to the airport, officials said.

The elite counterterrorism forces moved toward the Ghazlani military base where fierce clashes erupted at the edge of the base, two special forces officers said.

On Sunday, after weeks of preparations, 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 then, the military says 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.

In January, Iraqi authorities declared the eastern half of Mosul "fully liberated" from IS.

However, the streets are older and narrower in the western 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.