The first Western journalist to access territories controlled by terror group Islamic State (Isis) has said he spent time with Americans who left the US to join the insurgents.

Jürgen Todenhöfer, a renowned German journalist and former politician, was allowed to travel to Mosul, the largest Iraqi city occupied by the militants, after months of negotiations with some of the group's leaders.

In an interview with ABC news, the 74-year-old explained he met at least 14 American men who had joined the militants.

"They were very tough, well informed. Some of them very successful in their own countries. In Iraq 30% are foreign fighters; in Syria 70%. They feel discriminated in their countries," he said.

"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."

Todenhöfer then said that he has seen many wars and IS is "the strongest group I ever met. Very strong, very clever, very enthusiastic. They are extremely brutal. Not just head-cutting. I'm talking about the strategy of religious cleansing. That's their official philosophy. They are talking about 500 million people who have to die."

Todenhöfer spent most of his time in Mosul, but also visited Raqqa and Deir al-Zor, two areas where the militants have overthrown local authorities and imposed their own laws on controlled populations.

In Deir al-Zor, a mass grave containing more than 200 corpses believed to belong to members of the al-Sheitaat tribe was found in December amid allegations IS executed them.

In a previous interview with website Der tz, Todenhöfer said: "Each day, hundreds of willing fighters arrive from all over the world," and that the group is "much stronger than we believe".

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IS insurgents have killed thousands of people in Iraq and Syria since last July.

The terrorists, who control large swaths of the two countries and aim to establish an Islamic caliphate, often execute people who refuse to comply with their strict sharia laws imposed on the occupied areas.

IS is very active on social media, used to promote the group's propaganda and find new recruits.

A recent report by Saudi Arabian Sakina - an independent, non-governmental organisation created to engage in dialogue online as a way to combat internet radicalisation - revealed that IS and other terror groups write about 90 tweets per minute.

Another way through which IS lures potential fighters is by staging slave markets, where the militants sell women belonging to religious and ethnic minorities.