Police in Germany are in possession of files containing personal data on members of the extremist Islamic State group and believe them to be authentic.

The announcement by federal criminal police came after Sky News in Britain reported it had obtained 22,000 Islamic State files on the border with Turkey and Syria.

The files detailed IS fighters' real names, where they were from, telephone numbers, and even names of those who sponsored and recruited the militants.

The broadcaster said the files were passed on to them on a memory stick stolen from the head of Islamic State's internal security police by a former fighter who had grown disillusioned with the group.

Germany's Sueddeutsche Zeitung newspaper reported it had also obtained "dozens" of similar files on the Turkey-Syria border, where it said Islamic State files and videos were widely available from anti-IS Kurdish fighters and also members of the Islamic State group itself.

A spokeswoman for Germany's Bundeskriminalamt said her agency is currently evaluating the files.

She declined to say where the agency obtained the files, how many documents are involved and how long it has had them.

Sky reported that the documents are a collection of forms filled out by recruits when they were inducted into IS. The forms have 23 questions and include nationals from at least 51 countries, it is reported.

Germany's interior minister, Thomas de Maiziere, said that the material would give authorities a better chance to track down and prosecute people who had fought with IS.

The material also seems to have the potential to help authorities crack recruitment networks in Europe and elsewhere that have been sending fighters to join IS, which has seized large areas of land in Syria and Iraq and declared a self-styled caliphate on the territory under its control.