ERBIL, Iraq — Iraqi forces started storming the ISIS-held Old City of Mosul Sunday, an Iraqi military official confirmed to NBC News. It is believed to be the group's final stronghold in the city which used to be their capital in Iraq, according to Reuters.

The campaign to liberate Mosul entered its ninth month on Saturday and if the forces succeed in driving out the militant group from the city it will effectively spell the end of the caliphate in Iraq.

Photos: Iraqi Forces Storm ISIS-Held Old City Of Mosul

The Iraqi army, federal police and counter-terrorism troops launched the offensive on the Old City, said Brigadier Yahya Rasool spokesman for the Joint Operation Command.

An Iraqi military tank advances towards the Islamic State militants positions in the Old City in western Mosul, Iraq on Sunday. ERIK DE CASTRO / Reuters

The Old City is a densely-populated warren of narrow alleys where fighting is often done from house to house, according to Reuters.

Around 100,000 civilians remain trapped in the city, according to the United Nations.

Federal police were fighting around 150 meters away from the Great Mosque of al-Nuri, according to Lt. Col. Abdulrahman Al-Khazali, the police’s media spokesperson.

Related: Mosul Bridges Are Key to Liberating ISIS Stronghold

The mosque, with its famous leaning minaret, has become symbolic in the Iraqi military’s fight against the militant group, after ISIS leader Abu Bakr al-Baghdadi proclaimed a caliphate from the mosque in July 2014.

Lt. Col. Al-Khazali told NBC News that the police had advanced through the Old City from the south in order to recapture the mosque.

A general view shows smoke billowing from Mosul's Old City on Sunday, during the ongoing offensive by Iraqi forces to retake the last district still held by the Islamic State group. AHMAD AL-RUBAYE / AFP - Getty Images

Iraqi forces on the ground are being supported by the U.S.-led coalition as well as Iraqi war planes, Brigadier Rasool said.

The offensive follows months of fighting as Iraqi forces attempt to liberate the country’s second-largest city.

The wider campaign to drive out ISIS from Mosul began in November last year.

An NBC News producer reported from Erbil. Saphora Smith reported from London.