Fiirefighters put out a fire at oil wells, which were set ablaze by Islamic State militants before they fled the oil-producing region of Qayyara, Iraq, January 28,2017. REUTERS/Muhammad Hamed

BAGHDAD (Reuters) - Five oil wells are still burning out of 25 that Islamic State set on fire in Qayyara, south of Mosul, an oil ministry statement said on Thursday.

State-run North Oil Company crews are working to control the fires torched by the hardline militants to slow down the advance of U.S.-backed Iraqi forces toward Mosul, their last major city stronghold in Iraq.

The oil field was one of the main sources of revenue for the group that declared in 2014 a self-styled ‘’caliphate’’ in parts of Syria and Iraq.