An Islamic State leader has been killed in an airstrike in the northern Iraqi city of Mosul, residents and a local medical source said Thursday.

They said Radwan Taleb al-Hamdouni, whom they described as the radical militant group's leader in Mosul, was killed with his driver when their car was hit in a western district of the city on Wednesday afternoon.

The ultra-hardline Islamic State (aka ISIS or ISIL) swept through northern Iraq in June almost unopposed by Iraq's army, consolidating on gains made in the country's Sunni heartland region of Anbar.

Hamdouni was buried later Wednesday. Large numbers of supporters, some carrying black ISIS flags, attended the funeral, one source said.

He had been the ISIS "wali," or governor, of Mosul, which was captured by the group in June and remains the largest city in a self-declared Islamic State caliphate straddling the border between northern Iraq and eastern Syria.

The US, backed by some Western and Arab allies, launched airstrikes against the group in Iraq in August, later expanding operations to targets in Syria.

The air campaign, which Washington says aims to degrade ISIS' military capability, helped Kurdish forces retake territory from the group in Iraq and defend the Syrian border town of Kobane from an ongoing ISIS offensive.

ISIS fighters faced another setback this week when Iraqi officials said they had broken a five-month siege of the Baiji oil refinery — Iraq's largest — on Tuesday.

(Reporting by Dominic Evans and Saif Hamid; Editing by Michael Georgy/John Stonestreet/Susan Fenton)