Clad in red helmets and surgical masks, the firefighters emerged exhausted from the massive column of smoke streaming almost incessantly out of an oil well in northern Iraq.

They struggled since early morning to pump water into the well near Qayyarah, a small town in the Iraqi desert 60 kilometres (40 miles) south of second city Mosul.

Along with oil engineers and police officers, firefighters have been working around-the-clock to extinguish more than a dozen wells lit by Islamic State group jihadists in August.

IS fighters set the fields ablaze to slow government forces advancing on Qayyarah as part of their drive to wrest back Mosul from the clutches of the jihadists.

The wells have belched columns of toxic black smoke for three months, caking everything in and around Qayyarah in a thick layer of soot.

The firefighters' silhouettes were barely visible as they stood over the burning well near Qayyarah, where the mid-afternoon sun was dimmed by the black clouds overhead.

Trucks filled an adjacent reservoir with water, which was then pumped through a pipeline manually assembled by workers at the site into each well.

As they stepped out of the smoke, the firefighters joked with each other over the din of hammers and the loud whirring of the water pump, but said they were not authorised to speak to the press.

- 'Deplorable' situation -

Putting out the fires has proved to be a complex and dangerous process.

"First, the federal police have to check for mines left by IS at the mouth of the oil wells," said Saleh Khodr Ahmad, a worker at the site.

His blue jumpsuit was dotted with ink-black stains and he had tucked his thin surgical mask under his chin.

Once the area is cleared and the blaze brought under relative control, firefighters "place a pipe into the mouth of the well to pump water in, and cover it with dirt", said Ahmad.

The whole process can take up to a month and only two of the 19 lit oil wells have been extinguished.

"I'm exhausted, my body has been destroyed," Ahmad said.

According to the UN's environmental programme, crude oil fires "produce a wide range of pollutants, including soot and gases that cause health problems such as skin irritation and shortness of breath".

A pair of firetrucks and ambulances were parked at the ready in the sand nearby.

"The ambulances are here to treat the cases of suffocation and any injuries from explosives," said Ismail Ali Mohammed, a police officer tasked with guarding the site.

One of his colleagues was killed on Saturday after he stepped on a mine at the same well as firefighters were trying to bring under control.

"I've been here 15 days, working 24-hour shifts. We've all been poisoned by this smoke," Mohammed, whose family still lives in IS-controlled parts of Mosul, told AFP.

"This is a terrible scene, a deplorable situation."