Following seven months of negotiations with leaders of IS, Todenhöfer was the first western journalist allowed to travel to Mosul, where there are currently around 5,000 IS fighters.

Also accompanying him on his trip to the largest city occupied by the terrorist group was Todenhöfer's son who filmed the his father's unprecedented interviews with IS fighters.

Religious cleansing

Ahead of the publication of his "10 days in the Islamic State" on Monday, the 74-year-old told TZ that the support for IS in the region, which now has "dimensions larger than the UK," is "an almost ecstatic enthusiasm that I have never encountered in any other warzone."

According to Todenhöfer, hundreds of fighters are arriving to support the group every day from around the globe - something he described in the interview as "incomprehensible." Their main aim, Todenhöfer said, is to "plan the biggest religious cleansing in history."

Todenhöfer previously stayed in the same Benghazi hotel as James Foley, the US journalist who was beheaded by IS four months ago.

"Of course, I have seen the terrible, brutal video and it was one of my main concerns during the negotiations as to how I can avoid [the same end]," he told TZ.

During his visit to the "Islamic State," however, Todenhöfer slept amongst fighters on the floor or a plastic mattress in barracks formed from "the shells of bombed-out houses."

Increasing support

Due to the wide spread of fighters across the city, Todenhöfer said that were the US to bomb them, "they would have to reduce the whole of Mosul to ashes."

The journalist went on to say that Western intervention or air strikes will ultimately have little effect on IS, despite contrary reports from the US last week.

"With every bomb that is dropped and hits a civilian, the number of terrorists increases," Todenhöfer said.

In the interview, Todenhöfer also said that "without George W. Bush's Iraq war, there would be no IS."