An Iraqi officer says US-backed Iraqi forces are moving to surround Mosul's Old City a week after launching a fresh push to drive Islamic State militants from areas they still hold.

Iraqi special forces Lt. Gen. Sami al-Arathi said Thursday that battle plans changed and a northern advance was launched last week after Iraqi forces struggled to push into the Old City from the south.

The operation to retake Mosul began nearly seven months ago, and the eastern half of the city was retaken earlier this year. The battle for the more densely populated western half, including the Old City, has been slower.

The UN estimates some 350,000 people remain trapped in IS-held Mosul. Clashes over the past week have forced more than 11,000 civilians to flee. (AP)