"At 6:00 am we pushed deeper into the Old City and took control of new areas in the Faruq neighbourhood," he said.

Staff Major General Maan al-Saadi, a top commander in Iraq's elite Counter-Terrorism Service, told AFP that heavy fighting had resumed at dawn on Monday.

Commanders say the jihadists are putting up fierce resistance and there are fears for more than 100,000 civilians believed to be trapped in the maze of narrow streets.

Iraqi forces launched the operation Sunday to retake the district, the last part of Iraq's second city still held by IS after a months-long offensive.

Iraqi forces pushed deeper into Mosul's Old City on Monday after launching a final assault on the Islamic State group, warning civilians to stay inside and telling jihadists to "surrender or die".

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"Daesh resistance has been fierce," he said, using an Arabic acronym for the jihadist group.

"They have blocked every entrance, planted IEDs (improvised explosive devices) and booby trapped houses our forces might be near," he said. "Penetrating was very difficult. Today the fighting is face to face."

The push into Mosul's historic heart on t

he west bank of the river Tigris marks the culmination of a months-long campaign by Iraqi forces to retake IS's last major urban stronghold in the country.

The US-led coalition battling IS in Iraq and neighbouring Syria has backed the offensive including with months of air strikes.

The loss of Mosul would mark the effective end of the Iraqi portion of the cross-border "caliphate" IS declared in summer 2014 after seizing swathes of Iraq and Syria.

Loudspeakers blare warnings

s Old City on June 18, 2017 (photo by: AFP)

Iraqi forces stationed Humvees by the Grand Mosque on the retaken east side of Mosul, facing the Old City and mounted with speakers.

The loudspeakers blared messages to IS fighters, telling them: "You have only this choice: surrender or die".

Messages were also broadcast to civilians in the Old City saying Iraqi forces "are about to end your suffering".

Late on Sunday Iraqi forces dropped nearly 500,000 leaflets over the city, warning that they "have started attacking from all directions".

The leaflets urge civilians to "stay away from open places and... exploit any opportunity that arises during the fighting" to escape.

The United Nations has said IS may be holding more than 100,000 civilians as human shields in the Old City.

Commanders have said the fighting is expected to be very difficult and could last weeks.

Surrounded by Iraqi forces on three sides and blocked on the other by the Tigris River that runs through Mosul, the jihadists are cornered.

Iraqi forces launched a vast operation to retake Mosul in October, seizing the city's eastern side in January and starting an assault on its western part in February.

s second city still held by the Islamic State group, on June 18, 2017 (photo by: AHMAD AL-RUBAYE/AFP)

The International Rescue Committee, a major aid group operating in Iraq, has warned that already-traumatised civilians risk getting caught u

p in fierce street fighting.

It urged coalition and Iraqi forces to do "everything in their power" to keep civilians safe.

Fears for children

Save the Children warned that some 50,000 children were trapped in the Old City.

"They are running out of food and water, and face violence wherever they turn," the charity's Ana Locsin said.

"The impact of artillery and explosive weapons is likely to get even more deadly and indiscriminate, putting vulnerable children at greatest risk."

Since the start of the battle to retake Mosul, an estimated 862,000 people have been displaced. Around 195,000 have since returned, mainly to the city's east.

It was from the Old City's emblematic Al-Nuri mosque in July 2014 that IS chief Abu Bakr al-Baghdadi made his only public appearance.

He urged Muslims worldwide to move to the group's "caliphate" straddling Iraq and Syria.

s Old City on June 18, 2017 (photo by: MOHAMED EL-SHAHED/AFP)

The jihadists have since lost most of the territory they once controlled in the face of US-backed offensives in Iraq and in Syria, where a Kurdish-Arab alliance is advancing on the group's last Syrian stronghold Raqa.

It is not clear how many IS members remain in Mosul, where many foreign fighters have joined local jihadists since the city was taken.