Islamic State militants set fire to three oil wells near Hawija, west of the oil city of Kirkuk, one of two areas of Iraq still under their control, military and oil officials said on Monday.
Iraqi security forces were using bulldozers to control the fires started by the militants in the early hours of Saturday to slow the advance of U.S.-backed Iraqi forces and Shi'ite militia groups toward Hawija town, military officials said.
The Allas oilfield, 35 km (20 miles) south of Hawija, was one of the main sources of revenue for Islamic State, which in 2014 declared a caliphate 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," said army Colonel Mohammed al-Jabouri.
Military officials said the fire had been brought under control at one of the wells, while the other two were still burning. They said it would take about three days to put out the fires.
Oil officials from the state-run North Oil Company said it was still too risky to send its crews in to assess damage at the wells as militants may have left bombs and landmines.
Iraq launched an offensive on Sept. 21 to dislodge Islamic State from Hawija.
Islamic State, the ultra-hardline Sunni Muslim group, has lost control of all the oilfields it once controlled in the north of Iraq.