Two German-Russian men have been arrested on suspicion of spying for Russia and planning attacks in Germany -- including on US army targets -- to undermine military support for Ukraine, prosecutors said on Thursday.

The pair, identified only as Dieter S. and Alexander J., were arrested in Bayreuth in the southeastern state of Bavaria on Wednesday, federal prosecutors said in a statement.

The main accused, Dieter S., is alleged to have scouted potential targets for attacks, "including facilities of the US armed forces" stationed in Germany.

Police have searched both men's homes and places of work.

They are suspected of "having been active for a foreign intelligence service" in what prosecutors described as a "particularly serious case" of espionage.

Interior Minister Nancy Faeser likewise called the allegations "a particularly serious case of suspected agent activity for (Vladimir) Putin's criminal regime".

"We will continue to thwart such threat plans," she said, reiterating Germany's steadfast support for Ukraine.

Foreign Minister Annalena Baerbock summoned the Russian ambassador over the arrests, the ministry in Berlin said.

According to prosecutors, Dieter S. had been exchanging information with a person linked to Russian intelligence services since October 2023, discussing possible acts of sabotage.

"The actions were intended, in particular, to undermine the military support provided from Germany to Ukraine against the Russian aggression," prosecutors said.

The accused allegedly expressed readiness to "commit explosive and arson attacks mainly on military infrastructure and industrial sites in Germany".

Dieter S. collected information about potential targets, "including facilities of the US armed forces".

Fellow accused Alexander J. began assisting him from March 2024, they added.

Dieter S. scouted potential targets by taking photos and videos of military transport and equipment. He then allegedly shared the information with his contact person.

Der Spiegel magazine reported that the military facilities spied on included the US army base in Grafenwoehr in Bavaria.

"Among other things, there is an important military training area there where the US army trains Ukrainian soldiers, for example on Abrams battle tanks," Der Spiegel wrote.

Dieter S. faces an additional charge of belonging to a "foreign terrorist organisation". Prosecutors said they suspect he was a fighter in an armed unit of eastern Ukraine's self-proclaimed pro-Russian "People's Republic of Donetsk" in 2014-2016.

Germany is Ukraine's second-largest supplier of military aid, and news of the spy arrests came as Vice Chancellor Robert Habeck was on a visit to Kyiv. (AFP)