MOSUL, Iraq, Oct. 5 (Xinhua) -- U.S.-led coalition warplanes mistakenly struck pro-government fighters at a village in south of the city of Mosul, the last major Islamic State (IS) stronghold in Iraq, leaving 18 fighters killed and five others wounded, a provincial security source said on Wednesday.

The incident occurred on early Wednesday when the warplanes bombarded IS positions in the village of Haj Ali near the newly-freed town of Qayyara, some 550 km south of Mosul, but the airstrike mistakenly bombed the positions of the anti-IS paramilitary Sunni tribal fighters, the source told Xinhua on condition of anonymity.

The security forces launched a probe into the incident, while the troops and rescue teams are still looking for more bodies under debris of the destroyed posts, the source said.

The incident came as the Iraqi security forces and allied U.S.-led coalition forces are preparing for a major offensive to liberate Mosul, the capital of Iraq's northern province of Nineveh.

Mosul, some 400 km north of the Iraqi capital of Baghdad, is the second largest city in Iraq. The city has been under IS control since June 2014, when Iraqi government forces abandoned their weapons and fled, enabling IS militants to take control of parts of Iraq's northern and western regions.