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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 heart of their last major urban bastion in Iraq.

"Our forces are 800 meters (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 extremist-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.

Iraqi forces launched the huge operation last October, retaking the east of the city in January before setting their sights on the smaller but more densely populated west.

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 extremist rule.

Iraqi forces launched the huge operation last October, retaking the east of the city in January before setting their sights on the smaller but more densely populated west.

The Tigris River divides the two parts of Mosul. The Old City lies at the heart of the west bank.

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 Saturday.

That came after Jawdat announced the capture of the Al-Basha Mosque and the Bab al-Saray market Friday.

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.