Rapper-turned ISIS extremist didn't know he was married to FBI agent

A German rapper-turned ISIS extremist who starred in the group's grisly beheading videos was being spied on for the FBI by a woman he thought was his wife.

Denis Cuspert, 39, who now calls himself Abu Talha al-Amani, embraced radical Islam and travelled to Syria to fight with militants before becoming the group's main propagandist.

The spy transmitted critical information about the rapper and his ISIS colleagues before escaping to Turkey - where she was arrested and then turned over to the US, according to the German newspaper Bild.

The paper said German and American intelligence sources confirmed the existence of the operation, Fox News reports.

The unidentified woman fled Syria after her handlers told her it was no longer safe to continue the mission as militants had began to hunt for and flush out infiltrators.

She is now believed to be in the US.

German newspaper Bild believe Cuspert may have been targeted for the operation because of his womanising past.

In Germany he fathered three children by three different women, including one who he dumped just before embarking on his career as an ISIS poster boy.

Cuspert arrived in Syria in 2013 from a so-called German jihadi colony in Egypt.

It was not clear when he married the spy, but Bild reported that she informed her US handler via secret methods on regular basis about ISIS activities.

It is not known how she communicated the information to her handler.

Last week the US added rapper Cuspert to its list of "global terrorists" - freezing all his assets under US control and prohibiting any transactions with him in the future.

The State Department said: "Cuspert is emblematic of the type of foreign recruit ISIL seeks for its ranks -– individuals who have engaged in criminal activity in their home countries who then travel to Iraq and Syria to commit far worse crimes against the people of those countries."

In November, Cuspert was compared to Nazi Minister for Propaganda Joseph Goebbels for his ability to appeal to young, disillusioned people in Europe while providing an alternative ideology.

He spent several years as a major player in the Berlin hip hop scene under the name "Deso Dogg" before embracing radical Islam.

He has been known to oversee the group's sophisticated media operation and is reportedly using British fighters in an attempt to attract even more of their countrymen to the war-torn region.

It is understood he leads a unit of German-speaking ISIS terrorists operating under the name 'The German Brigade of Millatu Ibrahim'.

Last year, the rapper appeared in an ISIS beheading video and was seen holding the severed head of a man who he claims fought the terror group and as a result "received the death penalty".

Another man in the video explains in Arabic that the victims were members of Syria's al-Sheitaat tribe, a group of about 70,000 Sunnis who are fighting ISIS.

Around 700 members of the tribe were massacred by ISIS in August.

The Sun reports he is the subject of a dossier compiled by German security services, which quotes him as saying: "My duty is to use my voice for telling people the truth and the truth is jihad is a duty."

The lyrics in his song 'Who's Afraid of the Black Man' show his hatred for the West and his tendency towards violence.

The song states: "Doing time in my skin like Tookie Williams in San Quentin. No identity, where will this end?"

"In a white world full of hate and illusion. The last option was only violence and emotion."

Terrorism expert Raphael Perl of the Organization for Security and Cooperation in Europe (OSCE) told Dutch newspaperTrouw: "He gained fame with music and now he brings his fans in contact with a radical ideology."

The most recent German intelligence data estimates 500 German Muslims have traveled to the Iraqi and Syria.

Roughly 150 radical Islamic fighters are back in Germany, posing an enormous challenge for the country’s taxed counter-terrorism officials.