TAL AFAR, Iraq (AFP) - Seizing the city of Tal Afar district by district, Iraqi fighters would take down the Islamic State in Iraq and Syria (ISIS) group's black flags and hang them upside-down as they took "victory selfies".

But of all the areas they reclaimed, it was the historic heart of Tal Afar and its Ottoman-era citadel that was the high point.

Once an integral part of the Assyrian empire, Tal Afar's history goes back thousands of years and the city is dominated by the citadel, which was damaged in 2014 when ISIS blew up some of its walls.

The citadel "is a pillar of civilisation, it's a major historical monument for all the Iraqi and Arab people", said Abdel Hamid al-Attar, a 49-year-old fighter with the Hashed al-Shaabi paramilitary units that fought alongside government forces.

Atop a hill overlooking Tal Afar, the citadel weathered the many storms of violence that have shaken Iraq, including the 2003 US invasion that toppled Saddam Hussein.

After the invasion, it served as the headquarters of the municipal council and the local police.

Two years later, American forces set up a base in Tel Afar and launched Operation Restoring Rights to break the hold of Al-Qaeda and other insurgents in the city.

The operation was seen as a major success for the US military and was led by General H.R. McMaster, then a colonel and now US President Donald Trump's national security adviser.

Through it all, the citadel stood intact, until ISIS blew up its northern and western walls in 2014, sparking condemnation from UN cultural agency Unesco.

During their three-year occupation of Tal Afar, the extremist Sunni Muslim terrorists turned the citadel into a prison where they chained men and women whose behaviour they considered "sinful".

"When we retook the citadel, we found chains and other things ISIS used to restrain their prisoners," said Attar.

"I was shocked and sad when I saw the damage caused by IS," he said, using an alternative acronym for the extremist group.

Not far from the citadel stands Tal Afar's grand mosque, its minaret damaged during the fighting. Half-way up, Hashed fighters have hoisted the green banner of one of their units, the Abbas Brigade.

Fighters have taken up positions near the top to scout the area, and from up there they have a clear view of the extent of the damage inflicted on Tal Afar.

The offensive was preceded by intensive air strikes on ISIS targets and huge craters can be seen around the city, where electricity poles have been uprooted, homes and shops destroyed.

Some homes appear undamaged, but none of the militiamen dare go inside, fearing they could be booby-trapped.

Except for the fighters, there is not a soul around; most of the city's 200,000-strong residents were long gone before the offensive was launched last week.

ISIS graffiti is everywhere, however, with "Property of the Islamic State" or "God is greatest" scrawled on walls and building facades.

In the district surrounding the citadel, ISIS black flags that once flew from every corner have been brought down by the Iraqi forces and turned upside down.

A group of Hashed fighters grab a tattered ISIS flag as one of their comrades, who calls himself Abu Abbas, takes out his cell phone to snap a "victory selfie".

Abu Abbas ridiculed ISIS for its boasting that "the Islamic State (in Iraq and Syria) will stay on and persist".

"Where are they? I don't see any one of them here," said Abbas, who hails from the Shi'ite shrine city of Karbala in southern Iraq.

As temperatures soar, some fighters find a spring of cool water to wipe away the dust caking their faces and hair, while others simply dive in for a swim.

Suddenly, one of the Hashed militiamen shouts out: "Hey guys, do you think the water is booby-trapped?"