Everything is black here in the heart of darkness.

From children's hands to livestock to the claustrophobic sky, it is all covered in a thick layer of soot from the fires of hell that keep on burning.

The choking clouds of smoke come from oil wells set ablaze by the Islamic State as Iraqi forces battle to retake Mosul in the military's biggest operation for years.

The threat of death looms for shepherds who cannot sell their soot-blackened sheep. Photo / Getty Images
The threat of death looms for shepherds who cannot sell their soot-blackened sheep. Photo / Getty Images

Families have endured weeks of bombings and fierce fighting in the streets after state troops backed by allied forces began an operation to wrest back control of the country's second largest city.

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Yellow "suicide trucks" sit smouldering outside homes, with hundreds of families forced to flee as they struggle to breathe, and there is no respite for the parched, with up to 500,000 civilians facing a "catastrophic" drinking water shortage, according to the UN.

They are already coping without electricity and a severe lack of food, but the water crisis brings the spectre of mass deaths into sharp relief.

Shepherds have lost their livelihoods because they cannot sell sheep whose fleeces are matted with sticky oil.

A boy pauses on his bike as he passes a burning oilfield ahead of the Mosul offensive. Photo / Getty Images
A boy pauses on his bike as he passes a burning oilfield ahead of the Mosul offensive. Photo / Getty Images

The city has been under jihadi rule for two years. While coalition forces have made progress, returning families to areas south of Mosul and breaking into eastern parts of the city, many residents may not survive to see any so-called victory.

A major water pipeline has been damaged in an inaccessible part of the city controlled by IS, and locals are turning to unsafe water supplies, drinking from wells. And there is not even enough of this, Mosul residents told international news agency Agence France-Presse.

"Some people had stocks of dried goods but food is starting to run out, and we have neither water, nor electricity, nor fuel for heating," said 54-year-old Natiq as he queued for supplies in the eastern neighbourhood of Khadraa.

Thick, black clouds of smoke and oil cover people's homes and bodies as they fight for survival. Photo / Getty Images
Thick, black clouds of smoke and oil cover people's homes and bodies as they fight for survival. Photo / Getty Images

"Water is the most important thing, we aren't washing," said Iman Baker, a 34-year-old mother of three who lives in an eastern neighbourhood recently retaken from IS. "We are going to catch lice and our homes are filthy."

A medic at a hospital in the village of Gogjali on the eastern outskirts of Mosul said civilians were starting to arrive with "cases of diarrhoea and intestinal cramps, especially among children".

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Waterborne diseases such as severe diarrhoea and malnutrition could wipe out these vulnerable people living at the centre of the world's worst conflict.

Some residents blamed the US-led coalition backing Iraqi forces in the assault, saying its warplanes had damaged the main pipeline bringing water from the western side of the city.

The militants have been carrying out desperate suicide attacks and using civilians as human shields. Photo / AP
The militants have been carrying out desperate suicide attacks and using civilians as human shields. Photo / AP

But Basma Basseem, an official with the Mosul municipality, suggested IS may have intentionally stopped the flow.

"There are efforts to bring water tankers to neighbourhoods that have been retaken," she said.

Iraqi commanders say around 40 per cent of the eastern half of Mosul has been retaken in the offensive, and coalition air strikes have hampered IS's ability to launch suicide attacks across the city, according to British Army Major General Rupert Jones, a deputy commander for the US-led coalition.

Suicide missions, particularly those conducted in explosive-laden vehicles, have been a vital weapon for the jihadists. Iraqi forces told civilians to stay at home to avoid massive displacement from the city, but many have no choice but to run.

The progress of the forces - who vastly outnumber the estimated 3000 to 5000 jihadists defending their last major bastion in Iraq - has been slowed by the presence of a large civilian population often used by IS as human shields.

Sheep blackened by the soot of oil fires set by Islamic State militants graze in Qayara, 70 kilometres south of Mosul. Photo / AP
Sheep blackened by the soot of oil fires set by Islamic State militants graze in Qayara, 70 kilometres south of Mosul. Photo / AP

Almost 2000 members of the Iraqi forces and at least 926 civilians were killed just last month and 450 others wounded, monthly casualty figures from the UN mission in Iraq reveal.

After the launch of the offensive in mid-October, tens of thousands of Iraqi forces started closing in on Mosul, retaking towns and villages in Nineveh province.

The spike in casualties comes as a major offensive to retake Mosul, Iraq's largest military operation in years, enters its seventh week.

Isis has had control of the northern city for two years Pictured, Iraqi army humvees in the southern Mosul village of Haj Ali. Photo / AP
Isis has had control of the northern city for two years Pictured, Iraqi army humvees in the southern Mosul village of Haj Ali. Photo / AP

"The casualty figures are staggering, with civilians accounting for a significant number of the victims," the top UN envoy in Iraq, Jan Kubis, said.

Kubis said the growing death toll was largely a result of the jihadists' ferocious defence of Mosul, the city where they proclaimed their now crumbling "caliphate" in 2014.

Iraqi federal forces admitted they were met by stiffer than expected resistance when they entered the city on its eastern side, with military officials having warned the toughest fighting would be on the west.