The trial of Abu Walaa, named by authorities as Ahmad Abdulaziz Abdullah A., opened Tuesday, with proceedings due to be spread over 29 trial days until next January. Also facing charges of supporting IS are four co-defendants.

Armed police guarded the Upper Regional Court located in the historic city center of Celle, near Hanover, where the trial is taking place.

Abu Walaa, 33, nicknamed the "faceless preacher" for showing only his back to the camera in propaganda videos, arrived in Germany in 2001 as a teenager.

He was arrested last November on accusations of recruiting at least 16 young men for IS.

Six of them reportedly died on IS battlefields, mainly in Syria and Iraq.

He faces charges of belonging to a foreign terrorist organization and funding terrorism. The indictment said he had direct contacts in IS leadership circles.

The five accused could be jailed for 10 years.

Lessons in radical ideology

The alleged accomplices are a Turkish national, a German, a German-Serbian and a Cameroonian, aged between 27 and 51.

The eldest two, Hasan C. and Boban S., gave lessons in Arabic and radical Islamic ideology to prepare potential recruits for combat with IS, said the indictment.

Surnames are often not given in German coverage of trial proceedings.

Recruit turned informant

A lawyer for Abu Walaa has dismissed insider testimony, which, according to German news magazine Spiegel, was provided to authorities by another recruit who became disgusted with the terror group's tactics and turned informant.

Abu Walaa is also thought to have been in contact with Anis Amri, the rejected Tunisian asylum seeker who last December plowed a hijacked truck into a Berlin Christmas market, killing 12 people.

Read more: Abu Walaa: The 'faceless' Islamist preacher and the Berlin attack

Amri fled and was later shot dead by Italian police in Milan.

Germany's Der Spiegel news magazine said the trial in Celle promised to give insights into "background men” and those who "seduce and incite the jihad."

Previously, German media reports have alleged that Walaa's "students" included three teenage boys who last year detonated a home-made bomb at an Indian wedding in Germany, wounding a Sikh priest.

Base in Hildesheim

Walaa originally lived in the town of Tönisvorst in Germany's northwestern state of North-Rhine Westphalia before setting up his proselytizing base in Hildesheim in Lower Saxony state, 53 kilometers (33 miles) from Celle.

He delivered extremist sermons at a radical mosque, which has since been shut down.

German security services estimate that there are some 700 people (called Gefährder in German) in the country who represent a security risk and are capable of carrying out violent attacks.

ipj/se (dpa, AFP)