The militants, holed up in houses and darting through alleyways, resisted with sniper fire, suicide attacks and car bombs.

Iraqi government forces besieged ISIS militants around Mosul’s Old City on Thursday, edging closer to the historic mosque from where the group’s leader declared a caliphate nearly three years ago.

The militants, holed up in houses and darting through alleyways, resisted with sniper fire, suicide attacks and car bombs.

Though heavy rain hampered the advance, Federal Police and rapid response unit troops reached points about 500 m (yards) from the centuries-old al-Nuri Mosque by Thursday morning.

The black extremist flag was clearly visible draped from its famous leaning minaret.

The government forces have made significant gains in recent days in a battle that started in October, seizing a main bridge over the Tigris river and closing in on the mosque.

“We are holding positions we took yesterday. There is a lot of resistance in that area with snipers and car bombs,” Federal Police Major General Haider Dhirgham told Reuters.

The capture of al-Nuri Mosque would be a huge symbolic victory as well as a concrete gain.

“It’s important for them, it’s where they declared their state,” Dhirgham said, speaking at a police forward base as refugees trudged through the muddy streets and wrecked houses.

ISIS leader Abu Bakr al-Baghdadi chose the mosque as his backdrop for announcing the caliphate spanning Iraq and Syria in July 2014.

Since then, Mosul has been the hardline group’s main urban stronghold in Iraq but it has steadily lost ground since the offensive began. Iraqi leaders say the battle is reaching its final stages.

Several more areas of western Mosul had been recaptured, including the hospital, during Wednesday and Thursday morning but officers said progress was slowed by car bombs and booby-traps in houses and alleyways, as well as the bad weather.

ISIS hit back with sporadic attacks on government positions, including mortar fire. Suicide bombers had driven explosive-rigged cars at troops, Dhirgham said.

Government forces responded with mortars and helicopter gunships strafed militant positions from above.

Police said they had killed nine militants who tried to counter-attack one of their positions with rocket-propelled grenades.

“Federal Police drones bomb dozens of fixed and mobile ISIS targets in the perimeters of the Grand Mosque,” a later police statement said.