WATCH: Coalition warplanes destroy IS 'chemical weapons' factory

US-led coalition warplanes destroyed a factory in Iraq used by the Islamic State group to make chemical weapons, the Pentagon said on Tuesday.

The production centre - a converted pharmaceutical plant complex - was likely being used to make chlorine or mustard gas, said Lieutenant General Jeffrey Harrigian, who heads US Air Forces Central Command.

"This represents just another example of Daesh blatant disregard for international law and norms," he told Pentagon reporters in a video call.

The strike occurred on Monday near Mosul and was conducted by fighter jets, ground-attack craft and even a B-52 heavy bomber, the Pentagon said.

Observers have repeatedly alleged IS has used chemical weapons, and the Pentagon has confirmed the militants have deployed chlorine and sulphur mustard devices.

Iraqi security forces, backed by coalition air power, are in the final weeks of "shaping" operations ahead of an assault to recapture Mosul, which IS seized in 2014 and which remains the militants' last main stronghold in Iraq.