MOSUL, Iraq — Iraqi forces seized most of Mosul’s airport on Thursday, an important milestone in the broader offensive to retake the western half of the country’s second-largest city from the Islamic State, Iraqi and allied officials said.

The push to take the airport, which has been led by the Iraqi federal police, is a promising start to what is expected to be a difficult and bloody fight to completely evict the Islamic State from the city.

“They are most of the way through the airfield,” Brig. Gen. Matthew C. Isler, a senior United States Air Force officer in the American-led headquarters in Baghdad, said late Thursday afternoon.

It took Iraqi forces 100 days to seize the eastern half of the city, an operation that led to significant Iraqi casualties. Gen. Joseph L. Votel, the head of the United States Central Command, who arrived in Baghdad on Thursday, said about 500 Iraqi military personnel had been killed and about 3,000 had been wounded in that operation.