Mosul, Iraq - Tarik Hassan and his family became human shields 18 months ago. Fighters with the Islamic State of Iraq and the Levant (ISIL) came to their village home near the town of Hammam al-Alil and ordered the 30-year-old, his wife and five children to pack and leave.

From then on, they would live in Mosul, the Iraqi capital of ISIL's self-proclaimed caliphate.

The Hassans say that they were targeted because ISIL, also known as ISIS, knew that Tarik's cousin was a member of Iraq's national intelligence service. They joined kidnapped relatives of police and army officers held near ISIL fighters' bases and facilities as insurance against widespread use of heavy weapons or air strikes by the Iraqi army.

The more rattled the group's leaders became, the more they made use of this gambit, and when the operation to recapture Mosul began in October, fighters forced tens of thousands more civilians into the city and executed those who refused to comply, the United Nations said.

Mosul's east bank was fully recaptured by the Iraqi army more than a month ago. But west of the River Tigris that bisects the city remains largely ISIL-held.

The fighter told me, 'You will never leave this house, we will die here and so will you.' Abdul Malik, 16, Mosul resident

Efforts to retake these districts from ISIL intensified in February and as troops entered densely populated neighbourhoods, some human shields were able to escape to describe their ordeal.

Exhausted, Tarik and his family arrived at a military checkpoint and medical clinic on raised ground to Mosul's southwest on Wednesday. Nearby, others also told of being driven from their homes towards the city.

Mahmoud, 47, who pushed a handcart containing bags, several newborn lambs and his six-year-old nephew, said ISIL had at first tried to persuade villagers that they faced a grim fate at the hands of advancing troops, but then eventually forced anyone remaining out as well.

He added that ISIL told them that those who tried to stay would be "considered infidels" and killed as a result.

As many as 2,000 Mosul residents fled that day, appearing first as a line of tiny, dust-shrouded silhouettes trudging across a rocky ridge on the city's outskirts before eventually reaching a scorched asphalt road strewn with empty bullet casings.

They carried treasured possessions, infants or relatives too frail to make their own way, and waved white flags made from curtains and sheets.

ISIL mortar shells landed in the hills around them, provoking little reaction. All fell wide this time, although a young boy had been killed the previous morning, doctors said.

Iraqi forces continued a near-constant bombardment of the city - artillery rounds whined overhead and attack helicopters made low passes, raking targets with rocket and cannon fire.

The new arrivals were visibly scared and often gaunt. They spoke of hiding in basements for a week or more as fighting raged and supplies ran dangerously low. Some had survived on rotten bread or unsweetened tea for days. The checkpoint and clinic, which consisted of few field cots under battered blast walls and a giant ISIL billboard, marked safety at last.

"I can't believe I'm seeing you," said one woman, smiling through tears as foreign volunteer medics and soldiers distributed water next to scrawled graffiti reading "Make Mosul Great Again".

Further down the road, soldiers set up an assembly point where the displaced families crowded on to olive, flat-bed trucks that ferried them to reception centres and camps.

The majority were residents of Mosul's al-Maamoun neighbourhood. They had not been kidnapped, but many became hostages all the same. Abdul Malik, 16, described ISIL fighters parking an armoured vehicle in the courtyard of his house and insisting that he remain.

"The fighter told me, 'You will never leave this house, we will die here and so will you,'" he recalled. As ISIL fighters began to engage approaching Iraqi forces, however, he was able to slip away.

Others told of being instructed to move towards ISIL strongholds as the Iraqi troops pressed closer, adding that disobedience was met with lethal force. Fighters had appeared at 29-year-old Adnan Ghanim's house the previous night and ordered them to travel further into Mosul.

They ignored the instruction, however, and instead escaped with a group of neighbours as Iraqi soldiers appeared on the edges of their district.

He recounts the story while waiting cross-legged for transport to a camp and cradling his four-year-old son. Fear began to turn to relief as they made their way out of town, he said.

But then an ISIL suicide attacker in an explosive-packed vehicle suddenly appeared in the distance and swerved towards them. "We were happy that we escaped. But then we saw a car heading towards us," he recalled. It reached just a block away before the Iraqi army blew it up, causing only minor injuries to the group.

As he recounted this story, his son lay still in his arms, a smear of dried blood under one nostril, his eyes blank and staring. He is still traumatised by the explosion, Ghanim said.

READ MORE: ISIL 'using thousands as human shields' in Iraq's Mosul

Medical staff at the clinic described ISIL's brutal and concerted efforts to prevent civilians from fleeing. "Mainly when they try to escape they are being targeted by mortars," said the clinic commander, who spoke on the condition of anonymity. As many as 50 civilian casualties a day were treated in the clinic last week, many of them children.

Soldiers fighting in the city echo these accounts, saying that civilians are being kept inside their houses and forced to pull back when security forces advance. Major Mohammed Masood, 39, the commander of the Nineveh Province SWAT team, which contains many men from Mosul, says the family of one of his personal guards had been driven further and further into the city.

"They moved so many people," he said, speaking in a base outside eastern Mosul. "We don't have an exact number but we think between 200 and 350 families."

ISIL's use of human shields is not a new tactic, he added, but it has been used more and more as forces pressing on the west make greater use of heavy artillery.

The increasingly desperate fighters are now being herded into an ever smaller area as the fighting rages more intensely. For the thousands of hostages they have taken with them, the situation is likely to become increasingly perilous.