BAGHDAD - A German journalist who recently spent 10 days with Islamic State extremists in Iraq and Syria said he was surprised to find many were foreign fighters - including some Americans.

"The guy from New Jersey. This surprised me," Jurgen Todenhofer told ABC News after recently returning from the region. "You don't expect someone from New Jersey going to join the Islamic State."

Todenhofer, 74, who is working on a book about the group, wrote to 80 militants last year and developed a relationship with two that gained him and his son entry into their self-proclaimed caliphate, or Islamic state.

There the German writer met more than a dozen American fighters, including whites and Arab Americans, some of them born in the United States. He told ABC that foreign fighters composed 70 percent of the militants in Syria, 30 percent of the militants in Iraq.

"They were very tough, well-informed. Some of them very successful in their own countries," he told ABC, adding: "They feel discriminated in their countries. They want to live in an Islamic state where no one can bother them, and no one can discriminate against them. They are completely sure they will win this fight."

He told CNN that in the northern Iraqi city of Mosul, where he spent most of his time, he witnessed "an awful sense of normalcy" under Islamic State. "One hundred thirty thousand Christians have been evicted from the city; the Shia have fled; many people have been murdered, and yet the city is functioning, and people actually like the stability that the Islamic State has brought them."

At the same time, he added, "Many of them are quite scared, because the punishment for breaking the Islamic State's strict rules is very severe."

Todenhofer also visited Islamic State-controlled areas of Syria, including Raqqah and Dair Alzour. A veteran war correspondent who has covered al-Qaeda and the Taliban, he described Islamic State fighters to ABC as "the strongest group I ever met: very strong, very clever, very enthusiastic" and at the same time, "extremely brutal."

"Not just head-cutting - I'm talking about the strategy of religious cleansing," Todenhofer said, meaning the group intends to kill millions to establish their caliphate. "That's their official philosophy."