LONDON — Although the Iraqi military on Friday began leafleting areas of western Mosul still controlled by the Islamic State, warning people to evacuate because an offensive was imminent, the American military is not predicting a speedy resolution to the battle for the city.

Speaking at a news conference in London, Col. Ryan S. Dillon, the American coalition spokesman for Iraq and Syria, would not predict a timeline for the Iraqi military, supported by American advisers and air power, to finally oust the extremists. “I am confident they will retake Mosul,” he said. “This is inevitable. It will happen.”

Islamic State militants overran the city of more than one million, as well as other large stretches of the country, over just a few days in June 2014. More than two years later, in October 2016, Iraqi forces trained and advised by Americans and other coalition countries began an offensive to retake the city. Iraqi officials predicted success within a few months, but Islamic State fighters have managed to hold onto important parts of the city’s west side, including its historic center.

Colonel Dillon declined to predict whether the Iraqis would retake the city within a year of the start of their offensive.