The smoke from fires lit by Islamic State jihadists to provide cover from air strikes has painted the northern Iraqi sky black, providing a dramatic backdrop for the Mosul offensive.

The use of smoke in warfare is likely as old as war itself but the masks and technology available to Iraqi forces in this conflict leave civilians, especially children, the most vulnerable.

As forces closed in on their Mosul bastion, IS set fire to oil wells, torched tyres inside the city and set up a defence system around it that includes burning oil trenches to blind their enemy's air and satellite assets.

In the area of Al-Tina, south of Mosul, billows of white smoke from a sulphur plant torched by IS were brought rolling in by the wind, mixed at times with black plumes from blazing oil wells.

In the resulting haze, which limited vision to a few hundred metres (yards), dust-caked children played on the roadside.

"It blocks our chest," said Tiba, an 11-year-old girl wearing a blue dress and red headscarf. Anas, a seven-year-old boy with curly brown hair, said his throat was hurting.

According to a UN statement, 600 to 800 people have sought medical assistance because of the toxic cloud released by the sulphur plant fire.

Most of them were checked at a health centre in nearby Qayyarah but its chief doctor said several cases had to be transferred to a better equipped hospital nearby.

- Hundreds treated -

Two civilians are confirmed to have died from inhaling the sulphur fumes.

That fire was put out over the weekend but oil wells, some of which have been burning for months, are till ablaze.

Civilians living on the edges of Mosul in areas not yet retaken by Iraqi forces are also affected and have limited options for treatment.

A medic at Mosul's Jomhuri hospital, whom AFP contacted but can not name for security reasons, said a growing number of residents were checking themselves in with respiratory problems.

"Those who suffer the most are people with asthma, especially children and the elderly," the medic said. "We are doing what we can but the shortage of drugs at the hospital is getting worse."

Doctors told AFP that IS fighters are taking heavy casualties from the ongoing fighting and keeping most of the dwindling medical supplies for themselves.

Abu Thaer, who lives on the eastern outskirts of Mosul, brought his five-year-old son to Jomhuri hospital last week.

"My son has asthma and he is suffering a lot from the smoke," he said. "The drugs still available are expensive so I moved him here, where he is being treated in the oxygen room."

Up to 1.2 million people are believed to still be living in the city and Abu Thaer said some were trying to move away from the fires to less affected neighbourhoods.

- Limited military impact -

According to health and chemical weapon contamination experts from the International Committee of the Red Cross, the smoke clouds around Mosul were not of the most toxic kind.

"The more the oil is processed -- the more it will contain toxic fumes, but it would be less black," the ICRC told AFP.

"The most lethal and dangerous toxic chemicals are those we don't easily perceive with natural senses," it said.

When masks are not available, civilians should use a wet handkerchief to cover their mouth and nose, the ICRC said.

On satellite imagery, a dotting of black smudges obfuscate the Mosul battlefield but experts argue the jihadist tactic has limited impact besides obscuring the vision of drones.

"Burning oil wells does cause a localised nuisance, but it doesn't stop us from collecting intelligence using a variety of aerial and space platforms," said Colonel John Dorrian, spokesman of the US-led anti-IS coalition.

David Witty, an analyst and retired US special forces colonel, said the fires were mostly effective at "temporarily impeding ground, tactical operations as combatting forces draw close to each other".

"Smoke can greatly restrict close air support from attack helicopters, but less so for higher flying aircraft which already have GPS locations for targets," he said.

In history, Salaheddin, the 12th century Iraq-born founder of the Ayyubid dynasty, set dry grass on fire to disrupt his enemy during the battle of Hattin (in what is now Israel) to clinch a decisive victory against the Crusaders.