Thick black plumes of smoke carry for kilometres over the northern Iraqi landscape, blotting out the afternoon sun and making it feel like night.

Huge fires have been burning uncontrollably for more than two months near the town of Qayyarah, after retreating Isis (Islamic State) fighters set fire to its oilfields to provide cover from air strikes.

The Iraqi Army managed to recapture the town from the jihadists in August, but is still struggling to repair the damage left behind.

The apocalyptic scenes are only a taste of what is to come for the troops when they push towards Iraq's second city of Mosul 40km north, in a long-awaited offensive expected to begin this week. Thousands of Iraqi soldiers and US special forces were seen moving towards the front line near Qayyarah yesterday, in preparation for the country's biggest and most complex military operation since the 2003 invasion.

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From the skies, US-led coalition warplanes have been targeting Isis leaders and weapons facilities in and around the city to weaken the group.

A defeat in Mosul would be a serious blow to Isis' legitimacy.

The estimated 8000 jihadists in the city are expected to use everything in their arsenal to protect their last remaining stronghold in the country.

They have dug a 2m-by-2m trench around the perimeter of the city to be filled with burning oil once the army closes in. They are also thought to have rigged a chemical plant with explosives and plan to use the 1.5 million residents as human shields.

"Isis is panicked," one resident, Ahmed, said via a messaging app. "Since they lost Qayyarah, they have begun to tighten their security; carrying out mass arrests and raiding houses in search for weapons and illegal phones."

Residents face a choice between staying in Isis-controlled areas, where many have suffered extreme oppression and food shortages, or risking explosive devices and bullets to escape. Already more than 100,000 people have fled to about half a dozen pop-up tent cities. The UN is scrambling to build more, but if the numbers reach the highest predictions it could be too little too late.