"The army, counter-terrorism forces and federal police launched an attack on the Old City," Staff Lieutenant General Abdulamir Yarallah said in a statement.
Staff Lieutenant General Abdulwahab al-Saadi, a senior commander with the Counter-Terrorism Service, confirmed the "start of the assault on the Old City".
Iraqi forces backed by air strikes from a US-led coalition are pressing an offensive to retake the Old City on the west side of the city from the jihadists.
"The initial air strikes started at around midnight. The security forces started storming parts of the Old City at dawn," an officer with Nineveh operations command said.
Machinegun fire crackled and plumes of smoke from missiles rose above the Old City.
But across the Tigris river, on Mosul's east bank, life went on almost as usual as shoppers, students and workers pushed through traffic jams.
Taking back the Old City, a densely populated warren of narrow alleyways on the western side of Mosul, is crucial to recapturing the whole of the former IS bastion.
Iraqi forces launched the battle for Mosul in October, retaking the eastern part of the city in January and starting the operation for its western part the next month.
سایت تابناک از انتشار نظرات حاوی توهین و افترا و نوشته شده با حروف لاتین (فینگیلیش) معذور است.