More than 140 civilians have been killed in less than a week while trying to flee western Mosul, according to military sources, as the Iraqi army seeks to close in on fighters from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) in the armed group's last stronghold in Iraq.

Iraq: Almost one million people displaced from Mosul

According to the military on Thursday, most of the fatalities were women and children. 

More than seven months into a massive US-backed operation to retake Iraq's second city, security forces have recaptured all but a handful of areas of Mosul from ISIL.

Hundreds of thousands of civilians may be trapped in areas still held by the group, which seized Mosul in the summer of 2014.

Six days into a new assault on ISIL's remaining pockets of territory, the Iraqi army and their allies have progressed slower than they expected, Al Jazeera's Osama bin Javaid, reporting from Erbil in northern Iraq, said.

"They are facing fierce resistance from ISIL fighters," he said, adding that the civilian death toll had risen to 140 as of 13:00 GMT Thursday, amid the heavy clashes.

"It has just become more gruesome," he said. "It's becoming a tough fight. Iraqi forces are trying to hit ISIL targets shielding behind civilian homes, among densely populated areas."   

"Seventy civilians were reported to have died due to an air strike on Tuesday. It was not clear if it was Iraqi army or coalition forces who conducted the air strike."

Our correspondent also reported that ISIL fighters are using mobile mortar squads backed by snipers and suicide car bombs.

"It is a complicated battlefield that Iraqi forces are trying to navigate, and the highest price being paid is by the civilians of Mosul," he said.  

Reports said ISIL fighters have deployed mobile mortar squads and suicide cars bombs against Iraqi forces in Mosul [Reuters]    

750,000 displaced

Most of the civilians are in the Old City, which lies immediately south of where the current fighting is taking place.

The area, a warren of narrow streets and closely-spaced buildings, has posed a major challenge for security forces, and the coming battle to retake it poses a major threat to civilians.

"There are densely populated neighbourhoods," Javaid said. 

"Streets upon streets of people who are living in the besieged areas of Mosul. Around 200,000 people live there, and ISIL uses them as human shields." 

More than 750,000 people have been displaced since the start of the Mosul operation in October according to the UN, and that figure could increase sharply in the final stages of the battle for the city.

Around 150,000 of the displaced have since returned to their homes.

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The Mosul offensive has taken much longer than expected, with Iraqi government advances slowed by the need to avoid civilian casualties.

The fall of the city would, in effect, mark the end of the Iraqi half of the ''caliphate'' declared in 2014 over parts of Iraq and Syria by ISIL leader Abu Bakr al-Baghdadi, in a speech from a landmark mosque in Mosul's old city.

In Syria, Kurdish forces backed by US air raids are besieging ISIL forces in the city of Raqqa, the group's de facto capital in that country.

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Source: Al Jazeera and news agencies