Updated

ISIS militants set fire to three oil wells west of the city of Kirkuk, one of two areas of Iraq still under the terror group's control, officials said Monday.

Iraqi security forces were using bulldozers to control the fires started by the militants in the early hours of Saturday. The blazes began as an attempt to slow the advance of U.S.-backed Iraqi forces and militia groups, according to Reuters.

The Allas oilfield, 20 miles south of Hawija, was one of the main sources of revenue for ISIS, which declared a caliphate in 2014 in parts of Syria and Iraq.

“Terrorists are trying to use the rising smoke to avert air strikes while retreating from the area towards Hawija,” army Col. Mohammed al-Jabouri told Reuters.

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Military officials said the fire had been brought under control at one of the wells, but the other two were still burning.

It is still too risky to send its crews in to assess damage at the wells as militants may have left bombs and landmines, oil officials said.

Iraq launched an offensive on Sept. 21 to dislodge ISIS from Hawija.

ISIS has lost control of all the oilfields it once controlled in the north of Iraq.