Elite Iraqi forces said they were battling house by house in the Old City of Mosul on Saturday, inching towards the mosque where the Islamic State group proclaimed its "caliphate" in 2014.
Iraq began an operation on February 19 to retake west Mosul, which is the last major Islamic State group urban bastion in the country and includes the Old City.
Commanders said that progress in the densely populated warren of alleyways was slow, but that government forces had made new gains from IS in the ancient central district.
"Our forces are 800 metres (yards) from the mosque," said Captain Firas al-Zuwaidi, spokesman for the interior ministry's elite Rapid Response Force.
He was referring to the Al-Nuri Mosque, where IS leader Abu Bakr al-Baghdadi declared the cross-border "caliphate" spanning jihadist-controlled territory in Iraq and Syria in his sole public appearance in July 2014.
"We are encountering difficulties -- bad weather and streets too narrow for our military vehicles which cannot enter," Zuwaidi said.
"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 battle for the Old City was always expected to be the toughest of the campaign to retake Mosul from IS, further complicated by the presence of hundreds of thousands of civilians believed to have stayed on under jihadist rule.
- 'Hump of western operations' -
Iraqi forces in January retook the east of the city, which is divided by the Tigris River, before setting their sights on its smaller but more densely populated west bank.
The Old City lies at the heart of west Mosul.
Emily Anagnostos, an analyst from the Institute for the Study of War think tank, said the current phase of the operation was a delicate one.
"This stage is the hump of western operations that the ISF (Iraqi security forces) needs to get over without incurring significant ISF or civilian casualties," she said.
"ISIS resistance is tough in this area, the streets are too narrow for large vehicles, and the weather is poor. ISIS is exploiting these factors as part of their defence," Anagnostos said, using an alternative acronym for IS.
The Rapid Response Force is being backed up by the federal police who have made steady gains in recent days.
They have now taken the Al-Arbiaa market and a grain silo overlooking the Old City, federal police commander Lieutenant General Raed Shakir Jawdat said on Saturday.
That came after Jawdat announced the capture of the Al-Basha Mosque and the Bab al-Saray market on Friday.
"The federal police and the Rapid Response Force led a surprise attack and besieged Daesh cells in the Old City, killing 13 of them with grenades," Jawdat said, using an Arabic acronym for IS.
- Civilians evacuated -
Iraqi forces had already taken a string of key targets in west Mosul, including the airport, the train station, Mosul Museum and the provincial government headquarters.
The fall of Mosul, Iraq's second city, would be a major setback for 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 launched strikes against IS in Iraq and neighbouring Syria in 2014.
Jawdat said Saturday the federal police had evacuated civilians in the vicinity of the Old City.
More than 150,000 people have fled their homes in west Mosul, the Iraqi authorities say, of which two-thirds have found shelter in camps near the city where they receive food, blankets and foam mattresses.
The United Nations has warned that the exodus of tens of thousands of west Mosul residents could overwhelm aid groups trying to help them.
Lise Grande, the UN's humanitarian coordinator in Iraq, has said that any increase in the pace of the exodus could stretch aid groups "to the breaking point".