A German-Afghan man who worked for years as an interpreter and adviser for the German military went on trial Monday on charges of spying for Iranian intelligence.

The 51-year-old man, who has been identified only as Abdul S. in line with German privacy rules, is charged with "a particularly serious case" of treason and with breaching official secrecy laws in 18 cases.

Prosecutors have given few details of the case. Media and the public were excluded from the trial at the Koblenz state court before the indictment was read, the dpa news agency reported.

Presiding judge Thomas Bergmann said the trial would be held behind closed doors “until further notice” because of security concerns. The public was later allowed back into the courtroom, but further exclusions were expected during the course of the trial, which is scheduled to last until at least March 31.

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The man's wife, Asiea S., also a German-Afghan dual citizen, has been charged with being an accessory to treason. Prosecutors have said she supported his passing of secret documents to Iran from the beginning, without detailing the nature of that support.

Defense lawyer Ulrich Sommer said neither has responded to the charges. He said he has found “no direct evidence" to support them.