More than 10,000 documents and a huge amount of digital data were seized after the group was driven out of Manbij in northern Syria in August, according to Major General Rupert Jones, Sky News wrote.

"If we want to keep Britain safe, we need to deal with Daesh," he said, using another name for the extremist group.

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General Jones said they will be expecting fresh intelligence if the coalition retakes the Iraqi city of Mosul, where U.S.-backed Iraqi and Peshmerga forces have launched an offensive.

"It will be a labyrinth of intelligence and we need to get that into the hands of the intelligence agencies," he said.