BAGHDAD — Iraqi forces backed by the U.S.-led international coalition fought their way Thursday into a sprawling military base outside of Mosul and onto the grounds of the city’s airport, taking control of the runway amid fierce exchanges of fire with Islamic State group militants.

The two-pronged advance is part of a major assault that started earlier this week to drive IS from the western half of Mosul, Iraq’s second-largest city

Iraqi federal police units, backed by regular army forces, entered the airport Thursday morning, according to two police officials who said heavy clashes were underway hours later with IS militants hunkered down inside several airport buildings.

The officials said coalition troops were with the advancing forces, though they didn’t specify the nationalities of the foreign forces. They spoke to The Associated Press 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.