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The Iraqi army, backed by helicopters, advanced on western Mosul and pushed into the area around the al-Nuri mosque on Sunday.

Taking the mosque where Islamic State leader gave a much publicized sermon in July of 2014 is a symbolic victory for the government. But retaking the Old City of Mosul is riddled with challenges.

"It is one of the toughest areas, and in fact it is difficult, chaotic and very tight, but our forces managed to get out of their vehicles and enter the Old City," Lieutenant General Haidre Yusuf Abdulla, of the Iraqi Federal Police, said.

Militants are believed to be using suicide bombers to block the advance of government troops. Military officials say fighting in the densely populated Old City is extremely difficult.

"The challenges are, firstly, how to avoid hitting civilians because they (IS) are using them as human shields. And secondly, the area is difficult because it is in an ancient neighborhood and we use less heavy weapons," Brigadier-General Abbas Al-Juburi, a commander with the Iraqi Rapid Response Division, said.

Civilians are streaming out of western Mosul with meager possessions and people wounded by landmines or booby traps.

"There are bodies lying on the ground; they were killed by booby traps under the car at this corner. They passed and stepped on the wire. There are 15 dead bodies; they’ve started to rot. We wanted to take them, but the militants didn’t let us. They shot at us," Mosul resident Abu Dunia said.

The Iraqi government launched a massive U.S.-backed operation in October to retake Mosul from Islamic State. About 250,000 people have been displaced from the area since that battle began.

"The situation is terrible. Islamic State militants have destroyed us. There is no food. There is no bread. There is nothing," said a man fleeing Bab el Beit in West Mosul.

But those who have escaped feel lucky. More than a half-million civilians remain trapped by the militants in the Old City of Mosul.