Iraqi forces backed by helicopter strikes engaged in heavy fighting with jihadists on the outskirts of the Old City on Sunday as they pressed an offensive to recapture west Mosul.
The elite Rapid Response Force and Iraqi federal police attacked the Islamic State group militants with rifles, machineguns, mortar rounds and rockets a month after the west Mosul operation began.
The joint forces were around 100 metres (yards) south of Mosul's Iron Bridge, which has been destroyed along with other bridges that span the Tigris River that linked the city's eastern and western sides.
Helicopters circled overhead harrying IS with barrages of bullets and rocket fire in strikes aided by weather that was clearer than it had been in recent days, AFP correspondents said.
"The aim of the battle is to go past Al-Hadidi (Iron) Bridge northwards," Brigadier General Abbas al-Juburi of the Rapid Response units told AFP.
He said the operation was complicated by the presence of hundreds of thousands of civilians believed to have stayed on under jihadists rule.
"The difficulties are the presence of families, how to avoid opening fire on families who are used as human shields" by IS jihadists, Juburi said.
The battle for the densely populated Old City, with its warrens of alleyways, was always expected to be the toughest of the campaign to retake Mosul from IS.
In January Iraqi forces retook the east side of the city before setting their sights on the west.
At the heart of the Old City lies the Al-Nuri Mosque, where IS leader Abu Bakr al-Baghdadi in July 2014 proclaimed the IS "caliphate" that spans jihadist-controlled territory in Iraq and Syria.
It was Baghdadi's first public appearance and the capture of the mosque would be highly symbolic and strategic for the Iraqi forces who have in recent days taken several targets from IS.
On Saturday elite forces battled house by house in the Old City as they tried to inch towards the mosque, but were slowed by bad weather and the complicated effort of navigating the narrow streets.
"Our forces are 800 metres (yards) from the mosque," Captain Firas al-Zuwaidi, the spokesman for Rapid Response, said on Saturday.
"The fighting is street by street, house by house," he said, as the sound of mortar fire rang out from the heart of Iraq's second city.
The Rapid Response Force is being backed up by federal police who have made steady gains since Friday, including the Al-Arbiaa market and a grain silo overlooking the Old City.
The taking of Mosul, Iraq's second city, would deal a major setback to IS following months of losses in Iraq and neighbouring Syria.
Iraqi authorities launched the fight to retake Mosul from the jihadists on October 17 last year, with the support of the US-led coalition that has been carrying out strikes against IS in Iraq and neighbouring Syria since 2014.