TAL AFAR, Iraq (Reuters) - Iraqi government forces broke through Islamic State’s lines inside Tal Afar on Friday, reaching the old city center and the neighborhood around the Ottoman-era citadel, according to a military statement.

A military vehicle of the Counter Terrorism Service (CTS) is seen during the fight with the Islamic State militants in Tal Afar, Iraq, August 25, 2017. REUTERS/Thaier Al-Sudani

On the sixth day of the offensive, elite Iraqi units seized the northern neighborhoods of Nida’, Taliaa, Uruba, Nasr, and Saad, according to a statement from the Iraqi Joint Operations Command (JOC).

The Iraqi forces have seized about three quarters of the city since the offensive started in the early hours of Aug. 20, according to the latest JOC map, published on Friday evening. The militants remain in control of the northeast quarter.

Tal Afar lies on the supply route between Syria and the former Islamic State stronghold of Mosul, 80 km (50 miles) to the east. It has produced some of the militant group’s most senior commanders.

Tal Afar, which had a pre-war population of about 200,000, is the latest objective in the U.S.-backed war on Islamic State, following the recapture of Mosul after a nine-month campaign that left much of the city, the biggest in northern Iraq, in ruins.

The fall of Mosul effectively marked the end of the self-proclaimed caliphate Islamic State declared over parts of Syria and Iraq in 2014. Tal Afar was cut off from the rest of IS-held territory in June.

Up to 2,000 battle-hardened militants remain in Tal Afar,

according to U.S. and Iraqi military commanders. The number of civilians left in the city is between 10,000 and 20,000, according to the American military.

As in the battle for Mosul, civilians are suffering.

Waves of residents fled the city in the weeks before the battle started. Those remaining are threatened with death by the militants, who have held a tight grip there since 2014, according to aid organizations and residents who managed to flee.

On Tuesday, the U.N. refugee agency UNHCR said those who had fled were suffering from dehydration and exhaustion, having lived off unclean water and bread for three to four months. People were arriving at camps for displaced people with wounds from sniper fire and mine explosions.

Two mass graves, containing about 500 bodies, have been found in Badosh, on the road between Mosul and Tal Afar, another military statement said on Friday. The dead are believed to be inmates killed by the militants when they overran a prison in the area in 2014, it said.