Footage released on social media purportedly shows Islamic State (Isis) militants breaching the security perimeter around Iraq's Baiji oil refinery after clashes with security forces early on 13 April.
The refinery had been under siege for five months after Isis militants tore through Iraq last summer. Security forces drove the militants out of the nearby town of Baiji in November.
They subsequently withdrew, leaving a single supply line to the refinery, which the militants cut on 12 April, surrounding the complex once again.
The footage appears to show fire coming from three oil storage tanks that militants set alight, according to Salahuddin provincial council.
Salahuddin provincial council member Khazaal Hammad said the attack on the refinery was the "most violent yet".
Hammad said the situation was under control, although the refinery is within range of fire from outside the perimeter of the complex, which covers an area of around 20 square kilometers