BAGHDAD (AP) — Iraqi forces on Friday gained control of most of Mosul’s airport on the city’s southwestern edge and all of a sprawling military base next to it, Iraqi officials said — key milestones in the first phase of the battle to rout Islamic State militants from the western part of Mosul.

According to the spokesman of the Joint Military Operation Command, Brig. Gen. Yahya Rasool, after a day of fierce fighting, most of the airport was under Iraqi forces’ control.

A federal police officer said fighting was still underway in a small northern section of the airport, with pockets of IS militants there. Meanwhile, an Iraqi special forces official said his troops have retaken all of a sprawling military base adjacent to the airport. Both the federal police and special forces officials spoke on condition of anonymity in line with regulations.

The multi-pronged assault onto the base and the airport started at sunrise on Thursday, closely supported by U.S.-led coalition troops on the ground and airstrikes, as part of the battle to take the western half of Mosul that began six days ago.

Iraq’s special forces joined federal police and rapid response units in the push while the Popular Mobilization Forces — an umbrella group of government-sanctioned Shiite militias — secured the main roads west of Mosul, largely cutting the city off from IS-held territory in Syria.

Iraqi authorities declared Mosul’s eastern half “fully liberated” in January and afterward largely paused operations to prepare for the fight for the city’s west.

The United Nations estimated that about 750,000 civilians are trapped in western Mosul. The initial numbers of displaced from western Mosul have been low, but Iraqi forces are yet to punch into the city’s dense urban neighborhoods.

The battle for western Mosul, the extremist group’s last major urban bastion in Iraq, is expected to be the most daunting yet, according to Iraqi and coalition officials.

The western half of the city is denser with older neighborhoods and narrower streets that will likely complicate the already difficult urban combat ahead.