Iraqi army helicopters are firing rockets at ISIL positions in Mosul's Old City as troops on the ground close in on al-Nuri Mosque, a centuries-old structure famous for its leaning minaret.

Federal Police troops advanced past the train station in western Mosul closer to the mosque on Sunday. A police commander said they were very close to taking control of it.

Residents fled from the area, carrying bags of belongings and picking their way through the wrecked buildings as shells and gunfire echoed behind them. Most of them were women and children.

"Federal Police and Rapid Response forces resumed their advance after halting operations due to bad weather. The troops have a target of retaking the rest of the Old City," a police spokesman said.

OPINION: Mosul after ISIL - Insurgency and rivalry

The battle to recapture the last stronghold of ISIL, or the Islamic State of Iraq and the Levant group, in Iraq has now entered its sixth month.

Iraqi government forces, backed by US advisers, artillery and air support, have cleared the east and half of western Mosul and are now focused on controlling the Old City.

Recent fighting has targeted the strategic al-Nuri Mosque. Its capture would be a blow for ISIL, also known as ISIS, as it was from there that leader Abu Bakr al-Baghdadi declared a caliphate in July 2014 after the group had seized chunks of Iraq and Syria.

US officials estimate about 2,000 ISIL fighters remain inside Mosul, Iraq's second-largest city, resisting with mortar fire, snipers and suicide car bombs that attack army positions.

The black ISIL flag still flew from the mosque's minaret on Sunday.

Residents flee

Al Jazeera's Stefanie Dekker, reporting from east Mosul, said thousands of civilians were trying to escape the fighting.

"We have been hearing many stories from people feeling western Mosul - civilians caught in the middle, being killed by all sides in the conflict. The numbers seem alarmingly high. So far, the exact toll has been impossible to confirm," she said.

"As Iraqi forces push further into the heart of western Mosul, the more resistance they face, especially since the battle has now reached the outskirts of the densely populated Old City. ISIL is using a seemingly never ending supply of suicide car bombers and snipers."

As many as 600,000 civilians may be caught inside the city with ISIL fighters.

About 255,000 people have been displaced from Mosul and surrounding areas since October, including more than 100,000 since the military campaign in western Mosul began on February 19, according to UN figures.

The last week has seen the highest level of displacement yet, with 32,000 displaced between March 12 and 15.

PHOTO GALLERY: The battle for western Mosul

"The front lines haven't moved much over the last few days," our correspondent said.

"The weather was playing a role on this battlefield. When the sky is grey and it's raining, Iraqi advances are usually slowed, as the cloudy skies hinder pilots and their air support. ISIL also often uses the cover to carry out counterattacks.

"The weather also has an effect on the tens of thousands who are streaming out of Mosul, making their lives even more miserable."

Federal Police moved into the city on foot from near the train station towards the Old City, negotiating rubble-filled streets.

General Khalid al-Obedi, the police commander, said: "We are advancing toward the Old City. Their resistance is weakening. They are mostly using car bombs and that shows they are losing on the ground."

Meanwhile, Federal Police arrested Husam Sheet al-Jabouri, the local chief of Diwan al-Hisba, an ISIL unit responsible for enforcing strict Islamic rules, in Mosul's Bab al-Sijin area, a police statement said.