The Islamic State group (ISIS) is planning to use nuclear weapons to launch a global assault on anyone who holds differing religious beliefs – all in the name of worshipping Allah, according to German journalist Jurgen Todenhofer, who in 2014 was the first western reporter allowed to embed with the terrorist outfit and live to tell about it, spending 10 days in Mosul in northern Iraq with its fighters.

"The terrorists plan on killing several hundred million people," Todenhofer, 74, writes in his book, "Inside IS - Ten Days in the Islamic State," which details his findings, reported the U.K.'s Daily Express.

"The west is drastically underestimating the power of ISIS. ISIS intends to get its hands on nuclear weapons," he added, comparing the group to a "nuclear tsunami preparing the largest religious cleansing in history."

After serving as an MP in German Chancellor Angela Merkel's CDU party, Todenhofer became a journalist in 2000, specializing in war reporting, according to WND.

In 2014, he spent 10 days embedded with ISIS on the frontline, reportedly dodging bullets and death threats under the watch of the infamous beheader and England born "Jihadi John."

Todenhofer confirms in his book that ISIS "now control[s] land greater in size than the United Kingdom and are supported by an almost ecstatic enthusiasm the like of which I've never encountered before in a war zone. Every day hundreds of willing fighters from all over the world come," according to the Washington Times.

He warned, "They are the most brutal and most dangerous enemy I have ever seen in my life. I don't see anyone who has a real chance to stop them. Only Arabs can stop IS. I came back very pessimistic."

As the six world powers that negotiated July's nuclear agreement with Iran prepare to implement the deal in early 2016, some worry that Middle Eastern states are now actively working to develop nuclear weapons of their own.

"With more countries obtaining nukes, the greater the possibility terrorists could obtain a weapon, which could be used to attack Israel or the West," writes L. Todd Wood for the Times.