The West is “dramatically underestimating” the danger that Islamic State poses, says a German journalist who during a trip to areas under the organization’s control was permitted to interview some of its members.

The Islamic State, also known as ISIS or ISIL, “is larger, much stronger and more dangerous” than the West realizes, the reporter, Jurgen Todenhofer says. “In comparison to it, Al Qaida is a dwarf,” he posted on his Facebook page.

Two weeks ago Todenhofer toured the Syrian town of Raqqa and the northern Iraqi city of Mosul, spending 10 days with the organization’s fighters before returning home to Germany. He reports that Islamic State has succeeded in establishing a caliphate in these areas.

Other journalists have toured these areas, but Todenhofer’s report is the first by a reporter who visited there after Islamic State established what it set out to do.

The journalist, who is 74, says the area under Islamic State’s rule is larger than that of Great Britain.

The group’s supporters demonstrate “an almost ecstatic enthusiasm, more than in any other war zone,” he says. Hundreds of new recruits arrive daily from around the world to join the ranks, he says.

`Religious cleansing strategy’

Their objective is to implement the group’s mission, which Todenhofer calls “the greatest religious cleansing strategy that has ever been planned in human history.”

Todenhofer traveled there to gather data for a book he is writing on Islamic State. Despite repeated warnings, he decided to travel to the region shortly after the organization had executed journalists it had captured.

Todenhofer had previously stayed at the Benghazi hotel booked by James Foley, who was beheaded on camera by Islamic State four months ago.

“Obviously, I saw that atrocious video,” he says. “That was my main worry during my negotiations - how to avoid a similar fate,” he told the German website Der tz.

After seven months of negotiations via Skype with leaders of Islamic State, Todenhofer obtained an official travel permit to Mosul, where 5,000 ISIS fighters are based.

“I had in my hands a permit issued by the caliph” - Islamic State’s leader, Abu Bakr al-Baghdadi - “which guaranteed my safe passage,” Todenhofer says. “Unfortunately, I had no guarantee that this promise was secure.”

Todenhofer set out with his son, who acted as photographer, documenting his unique interviews with Islamic State fighters.

During his visit to Islamic State, Todenhofer slept side by side with the fighters in their dwellings, which are actually the “bombed out shells of houses.”

Looking back on his decision to go, he says he “knew that the risks were high.” The risks also included bombing by Syria and the U.S., he added.

Referring to the bombing campaign carried out by the U.S. and its allies, he said that every bomb that hits civilians swells Islamic State’s ranks.

Islamic State provides welfare support and education to residents of towns he visited, he says. Islamic State’s presence in Mosul is so widespread that the U.S. would have to reduce the city to ruins to oust the group, he says.