Troops are pushing into the last Islamic State stronghold in Iraq's second largest city.
Iraqi forces have begun storming the Islamic State-held Old City of Mosul, in an assault they hope will be the last in the eight-month-old campaign to seize the militants' stronghold.
The historic district, and a tiny area to its north, are the only parts of the city still under control of the Islamists. Mosul used to be the Iraqi capital of the group, also known as ISIS.
The Iraqi army estimates the number of Islamic State fighters at no more than 300, down from nearly 6000 in the city when the battle of Mosul started on October 17.
But the Old City is a densely-populated maze of narrow alleyways and the fighting is slow, bloody and house-to-house.
About 100,000 civilians are trapped, with little food, water or medical treatment.
A US-led international coalition is providing air and ground support to the campaign.
Several air strikes during the day hit a medical complex just north of the Old City, alongside the western bank of the Tigris river, a Reuters TV reporter said.
The medical complex, housing the two biggest hospitals of Mosul, is still held in part by the militants who are using its buildings as sniper outposts.
Islamic State's security services chief in the Old City, Kanaan Jiyad Abdullah, also known as Abu Amna, was killed in the morning clashes, said Hisham al-Hashimi, who advises several Middle East governments on Islamic State affairs.
A high-ranking Islamic State figure in charge of intelligence in Mosul, Shakir Mahmud Hamad, was captured by the advancing troops in the Old City, Hashimi told Reuters.
The Iraqi government initially hoped to take Mosul by the end of 2016, but the campaign took longer as militants reinforced positions in civilian areas to fight back.
Hundreds of civilians fleeing the Old City have been killed in the past three weeks, as Iraqi forces could not fully secure exit corridors.
"An estimated 50,000 children are in grave danger as the fighting in Mosul enters what is likely to be its deadliest phase yet," Save the Children warned in a statement.
Islamic State snipers are shooting at families trying to flee on foot or by boat across the Tigris River, as part of a tactic to keep civilians as human shields, the United Nations said on Friday.
The fall of Mosul would, in effect, mark the end of the Iraqi half of the "caliphate" that Islamic State leader Abu Bakr al-Baghdadi declared in a speech from a mosque in the Old City three years ago and which once covered large areas of Iraq and Syria.