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Black smoke continues to billow into the air from the Qayyarah oil field, damaged by IS militants last month as they fled the town, creating health risks for civilians and troops amassing there.

The fires are also clogging up the skies in the area, where critically important airstrikes and aerial reconnaissance missions are taking place almost daily.

Located on the west bank of the Tigris River, about 65 kilometers south of Mosul, Qayyarah has since become an important staging ground for military and humanitarian efforts ahead of the Mosul operation since it was recaptured by Iraqi forces last month.

"Stabilising Qayyarah can't wait - it has to happen now," Lise Grande, the UN's humanitarian coordinator for Iraq, told AP.

"Everything for the Mosul operation hinges on Qayyarah. It's the staging ground for military forces and it’s where 350,000 of the 1 million people who are expected to flee [Mosul] will either find shelter or pass through."

There are slow-going Iraqi efforts to contain the fires, but nearly a month after the town was recaptured from the militants, smoke and toxic fumes continue to pollute the air in and around Qayyarah.

Nearly a month after the town was recaptured from the militants, smoke and toxic fumes continue to pollute the air in and around Qayyarah

The Iraqi Oil Ministry spokesman, Assem Jihad, said on Wednesday that IS militants set fire to 11 oil wells in Qayyarah to derail security forces and wreak havoc in the area as they fled.

He said fires at nine of the wells have been extinguished, but two continue to burn powerfully.

The images of smoke and flames from the oil wells are reminiscent of the oil fires in Kuwait after the Iraqi military reportedly set fire to hundreds of wells when Saddam Hussein invaded the neighboring Persian Gulf nation in the early 1990s.

"In putting out the fires in Kuwait the firefighters used water pipes and pumped the water from the Gulf to spray at the base of the fires," said Kourosh Kian, an expert in petroleum drilling and reservoir engineering.

Kian, a system engineer at GE Aviation, said the simplest method to extinguish these types of fires is to inject water under high pressure at the base of the fire.

Since Qayyarah is on the Tigris River, there would be no problem with the water supply, he said.

The two main fields in the area, Qayyarah and Najmah, had been producing about 30,000 barrels per day of crude before Daesh took control of Iraq’s Ninevah Province in June 2014.

The simplest method to extinguish these types of fires is to inject water under high pressure at the base of the fire

While Iraqi forces now remain in control of the area, it is far from stable.

At the Qayyarah west air base, where hundreds of US troops are working to advise and assist their Iraqi counterparts, a small rocket that contained a mustard agent landed, General Joseph Dunford, chairman of the Joint Chiefs of Staff, told Congress on Thursday.

A US official, who discussed details of the incident on condition of anonymity, said a small group of US soldiers who inspected remnants of the rocket after it exploded found a black, oily substance on a fragment of metal.

An initial test of the suspicious substance showed it contained residue of mustard agent, but a second test was negative.

The Iraqi military, backed by coalition airstrikes and coalition advise-and-assist operations, looks to recapture more territory from the military group, which at one point in 2014 controlled about a third of Iraq and neighboring Syria.

US-led coalition forces have launched more than 460 airstrikes around Qayyarah since August 2014 and more than 1,800 around the city of Mosul itself.