The last Islamic State militants in Mosul are reportedly putting up fierce resistance as Iraqi forces try to seize their final stronghold in the city.
One commander said penetrating the jihadists' defences was very difficult, as they had blocked all entrances to the area and booby-trapped houses.
Overnight, aircraft dropped leaflets urging civilians to avoid open spaces and to take any opportunity to escape.
The UN says IS may be holding more than 100,000 people there as human shields.
Pro-government forces launched an offensive to retake Mosul in October with air and ground support from a US-led coalition.
Como this The battle so far
On Sunday, commanders announced the start of the "final chapter" of the offensive, with Counter-Terrorism Service, Army and Federal Police attacking the Old City from all directions.
The army believes that there are no more than 300 militants left in Mosul, compared with almost 6,000 at the start of the offensive in mid-October
However, the densely populated Old City's narrow streets - Mosul's historic heart - mean the assault will be a major challenge, with troops having to clear the area house by house.
On Monday, Gen Maan al-Saadi of the Counter-Terrorism Service told AFP news agency his troops had made progress in the Farouq district, but that resistance had been "fierce".
"Penetrating was very difficult," he said. "Today the fighting is face to face.
