Abdul S., 51, appeared at the higher regional court of Koblenz in western Germany accused of "a particularly serious case of treason" and of "violating state secrets" in 18 instances.

He was arrested in January 2019, reportedly after a tip-off from abroad and an ensuing set-up to catch him in the act.

His German-Afghan wife Asiea S., 40, appeared in the dock alongside him, after prosecutors in December charged her with aiding and abetting treason.

She stands accused of helping her husband "with small organisational things", federal prosecutor Ullrich Schultheis was quoted as saying in court by DPA news agency.

Kabul-born Abdul S. worked for several years as a civilian translator and cultural adviser to the German Bundeswehr at the Heinrich-Hertz barracks in the town of Daun, near Koblenz.

He is suspected of receiving some 60,000 euros ($66,000) for passing on military secrets to Iranian intelligence services, including at times "explosive information", Schultheis said.

Officials have been tight-lipped about the case, releasing no details about the kind of information that was allegedly leaked.

The trial is taking place largely behind closed doors and is covered by strict and rarely used confidentiality procedures to protect state secrecy, the court said.

Abdul S. risks life in jail if found guilty, which in Germany usually means a sentence of at least 15 years.

His wife faces a maximum of 11 years in prison.

The couple have yet to respond to the accusations against them, their lawyer Ulrich Sommer said, according to DPA.

- Taking the bait -

According to Der Spiegel weekly, the hunt for the spy started in 2017 after Germany's military counterintelligence service (MAD) received a warning from "a friendly secret service" that Iran had an informant in the Bundeswehr.

MAD spy-catchers quickly focussed their attention on Abdul S. after noticing that his trips to certain EU cities overlapped with those of a senior officer in Iran's secret service, Spiegel wrote in an in-depth article last year.

To confirm their suspicions, the MAD started to drip-feed Abdul S. fake documents that appeared to contain sensitive or classified information, and then watched as he reached out to his Iranian contact to set up meetings.

The MAD shared its findings with federal prosecutors, tasked with handling cases of national security, in the spring of 2018.

Spiegel said Abdul S.'s work for the German military included eavesdropping on phone calls or intercepted radio messages from the Taliban.

But it "is doubtful" that Abdul S. had access to classified information about the actions and deployments of German troops in Afghanistan, the weekly added.

The case is embarrassing for the Bundeswehr since Abdul S. would have had to pass stringent background checks before being hired.

Germany's BfV domestic intelligence agency has identified Iran has one of the countries most active in spying on Germany, along with China and Russia.

Iranian spy services "are regularly looking for appropriate sources to cover the information needs of the regime", the BfV said in a report.

In 2018, Germany arrested a Vienna-based Iranian diplomat suspected of being a spy, with prosecutors alleging he was plotting with a Belgium-based couple to bomb an Iranian opposition rally in Paris.

In another high-profile case, former German intelligence agent Markus Reichel was convicted in 2016 for spying for both the CIA and the Russian secret service.

In 2011, Germany jailed a married couple for spying for the Russian secret services for more than 20 years.