Footage of air strikes on Monday showed 12 aircraft from the US-led coalition bombarding a large industrial site in Mosul.
Huge explosions could be seen hitting multiple buildings and storage units, leaving them in flames with large plumes of smoke rising into the air.
Isis released a propaganda statement claiming American fighter jets had destroyed the “only pharmaceutical company in northern Mosul” but defence officials said jihadists had converted the factory to manufacture chemical weapons.
Lieutenant General Jeffrey Harrigian, from US Air Forces Central Command said the facility posed a “significant chemical threat to innocent Iraqis”.
“Intelligence had indicated that Daesh [Isis] converted a pharmaceutical plant complex into a chemical weapons productions capability,” he added, saying 50 individual targets were hit.
“This represents just another example of Daesh blatant disregard for international law and norms.
“The enemy is using innocent civilians as shields against our values and respect for human life.”
United Nations investigators have confirmed Isis’ use of mustard gas in both Syria and Iraq and the group has also been accused of using chlorine gas.
Both chemical weapons are toxic and can kill their victims, frequently causing breathing difficulties, blisters and internal injuries.
Speaking from the Al-Udeid Air Base in Qatar, Lt Gen Harrigian said the US Air Force would continue efforts to “dismantle Daesh” and accelerate ground operations to drive Isis out of Mosul and Raqqa.
“We will continue to shape the battle space, going after their revenue streams, killing their leaders and creating organisational dysfunction,” he added.
“We will seek to use the weight of air power to remove Daesh’s legitimacy, shatter their vision and enable taking back the territory and resources they have stolen.”
Mosul is Isis’ de-facto capital in Iraq and its only city stronghold remaining in the country after militants were driven out of Fallujah and Tikrit.
The Iraqi security forces, supported by Shia militias and the international coalition, have been advancing on the city for months in preparation for an all-out assault.
Air strikes have recently targeted Isis bomb-making facilities and oil tankers in preparation for the operation, which the UN has warned could trigger a “massive” humanitarian crisis.
British air strikes have recently been focused on the town of Qayyarah – 40 miles from Mosul – which was re-taken by Iraqi forces last month after two years under Isis control.
In the past week, RAF fighter jets and drones have targeted Isis vehicles, a bomb factory, training facility, artillery and rocket and mortar teams in the region.
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