SOUTH OF MOSUL, Iraq — Closely supported by the U.S.-led international coalition, Iraqi forces secured a series of cautious advances Thursday, pushing into a sprawling military base outside of Mosul and onto the grounds of the city’s airport, where they took control of the runway.

The three-pronged attack began just after sunrise, with three convoys of Iraqi forces snaking north across Nineveh’s hilly desert on Mosul’s southern approach. Iraq’s special forces joined federal police and rapid response units in the push – part of a major assault that started this week to drive the Islamic State from the western half of Iraq’s second-largest city. By afternoon they had entered the Ghazlani military base south of the city, as well as the airport.

Iraqi helicopters circled above Mosul firing down onto the city’s southwestern edge. Coalition and Iraqi airstrikes that hit targets inside Mosul sent plumes of white smoke into the air on the horizon.

“We’ve broken the first line of IS defenses,” said Iraqi special forces Lt. Yaser Mohsen, whose troops captured the key village of Tell al-Rayan, where Islamic State snipers had been slowing the government offensive. They then moved to the edge of Mosul’s western Mamun neighborhood, where they were working to surround it before punching into the city.

Several armored coalition vehicles could be seen in the line of military vehicles, and security officials said coalition troops were embedded with the forward advancing forces, advising the Iraqi troops as they conducted the assault.

The cautious advance stood in sharp contrast to the first days of Iraq’s push into Mosul from the east, when Iraqi forces quickly advanced deep into the city’s congested neighborhoods, where they were hit with heavy counterattacks, including dozens of car bombs that struck the slow-moving Iraqi convoys with deadly consequences.

Clashes at Mosul’s airport continued for hours, with militants hunkered down inside several airport buildings.