U.S.-backed Iraqi federal police forces – backed by U.S. air support – entered Mosul International Airport and took control of the runway from the Islamic State. An official said Thursday they had been advancing on Mosul by assaulting the airport and launching raids on the Ghazlani military base, located on the southern outskirts of the city.

The Iraqi forces launched the offensive on Sunday to capture the west side of Mosul that remains under the control of the militants. Iraq declared eastern Mosul “fully liberated” last month after three months of fierce fighting, but fighting remains.

“We can confirm that the Mosul airport militarily has fallen and it’s a matter of short time to fully control it,” Sabah al-Numan, a spokesman for the Iraqi military, told Iraqia TV.

Mosul, Iraq’s second-largest city, fell to ISIS in the summer of 2014, along with large swaths of territory in northern and western Iraq. It was estimated 1,000 to 3,000 ISIS fighters were in the city.

Lt. Gen. Stephen Townsend said Monday the Iraqi forces are becoming an “increasingly capable, formidable and professional force.”

“Mosul would be a tough fight for any army in the world, and the Iraqi forces have risen to the challenge,” Lt. Gen. Townsend said in a statement.

But the fight against ISIS is far from over. In the eastern part of the city, yet militants have continued to target the area for attacks, including two suicide bombings against government forces on Sunday.

The battle for western Mosul could prove even more difficult. Roughly half of the city west of the Tigris River has older, narrower streets and is more heavily populated. Larger populations mean larger suffering and more media attention.

“The situation is distressing. People, right now, are in trouble,” Lise Grande, the U.N. humanitarian coordinator for Iraq, said in a statement. “We are hearing reports of parents struggling to feed their children and to heat their homes.”

Prime Minister Haider al-Abadi announced the start of the latest operation on state TV and said government forces were moving to “liberate the people of Mosul from [ISIS] oppression and terrorism forever.”