Iraqi troops have pushed into the last Islamic State stronghold in Mosul, formally launching the final major battle of an eight-month campaign to drive the militants out of the city.
The IS group captured Mosul when it swept across northern and central Iraq in the summer of 2014. Iraq launched a massive operation to retake the city last October, and has driven the militants from all but a handful of neighbourhoods.
The extremists are expected to make their last stand in the Old City, a densely populated quarter with narrow, winding alleys.
Lieutenant General Abdul-Amir Rasheed Yar Allah, who commands army operations in Ninevah province, said Iraqi special forces, the regular army and federal police are taking part in the operation to retake the Old City, which began at dawn on Sunday.
Iraq state TV aired live footage showing thick black smoke rising from the Old City, with the sound of gunfire rattling inside. It said leaflets have been distributed urging civilians to leave through five "safe corridors".
General Abdel Ghani al-Asadi, the head of Iraq's special forces, said he expects the extremists to put up a "vicious and tough fight", and his troops "will be very careful" to protect civilians in the densely-populated area.
The International Rescue Committee has called on Iraqi forces and the US-led coalition to "do everything in their power to keep civilians safe during these final stages of the battle for Mosul".
Nora Love, the aid group's acting country director, said: "With its narrow and winding streets, Iraqi forces will be even more reliant on air strikes despite the difficulty in identifying civilians sheltering in buildings and the increased risk of civilians being used as human shields by IS fighters.
She warned the assault could lead to even more civilian deaths than the hundreds killed so far in air strikes across the rest of the city, as "the buildings of the old town are particularly vulnerable to collapse even if they aren't directly targeted".
Those who try fleeing to government-controlled areas risk being caught in the crossfire or targeted by IS snipers, she added.
The Old City is home to the centuries-old al-Nuri mosque, where IS leader Abu Bakr al-Baghdadi delivered a Friday sermon in 2014 as his group declared an Islamic caliphate in the areas it controlled in Syria and Iraq.
The militants have lost much of that territory over the last three years, and Mosul is their last urban bastion in Iraq.
Up to 150,000 civilians are believed to be trapped in the Old City.