Obama said the push to liberate the northern city, the jihadists' last stronghold in Iraq, now in its second day, was a "major step forward."
"I am confident that ISIL will be defeated in Mosul and that will be another step toward their ultimate destruction," he told a joint news conference with Italian Prime Minister Matteo Renzi, using an acronym for the jihadist group. But the US leader warned of a challenging road ahead.
"Mosul will be a difficult fight. There will be advances and there will be setbacks," Obama said. "Perhaps one million civilians are still living there," Obama added.
"In addition to rooting out ISIL, our focus is on the safety and humanitarian aid for civilians escaping the fight. That will be a top priority for both our governments."