By Paul Kirby, BBC News
Two alleged spies suspected of planning to sabotage German military aid for Ukraine have been arrested in the southern German state of Bavaria.
The two men, described as dual German-Russian nationals, were detained in Bayreuth on suspicion of spying for Russia, prosecutors say.
Interior Minister Nancy Faeser said authorities had prevented "possible explosive attacks".
The men are accused of scouting US military facilities and other sites.
The main suspect, Dieter S, has been remanded in pre-trial detention accused of a string of offences, including plotting an explosion, arson and maintaining contact with Russian intelligence.
He is also alleged to have fought for a Russian proxy armed force in occupied eastern Ukraine from 2014-16.
The second suspect, identified as Alexander J, is accused of helping him since last month to identify potential targets for attack. He was due to appear in court on Thursday.
The Russian ambassador has been summoned by the foreign ministry in Berlin. Faeser condemned "a particularly serious case of alleged spy activity for [Russian President Vladimir] Putin's criminal regime".
Germany is the second largest donor of military aid to Ukraine after the US, earmarking some €28 billion (NZ$51b) since the start of Russia's full-scale invasion in February 2022.
According to prosecutors, Dieter S is alleged to have discussed potential sabotage operations in Germany with his Russian contact since October last year, in an attempt to undermine its support for Ukraine.
They cite preparing explosive and arson attacks, especially on military and industrial infrastructure. Dieter S is said to have scouted potential targets including US military facilities, taking photos and videos and handing the information to the Russian contact.
According to the Spiegel website, a US Army facility at Grafenwöhr in Bavaria was spied on. Last year, the US sent dozens of Abrams battle tanks to Bavaria for Ukrainian soldiers to train on at Grafenwöhr and another base at Hohenfels before the tanks were sent to the front line in Ukraine.
The case is reminiscent of series of arrests in Poland a year ago, when authorities said they had dismantled a Russian spy network which was preparing sabotage attacks aimed at paralysing supplies of military aid to Ukraine.
German Justice Minister Marco Buschmann told German news agency DPA that the two arrests were "another significant investigative success" in the fight against Russian sabotage and spy networks.
Vice-Chancellor Robert Habeck, currently on an unannounced visit to Kyiv, said on Thursday that he was there "at a time when Ukraine needed all the support it can get in its fight for freedom".
The government in Berlin is spearheading a plan to help bolster Ukraine's air defences.
Chancellor Olaf Scholz said on Wednesday that EU countries should try to help Ukraine with additional air defence systems, especially Patriot systems. Germany has already sent Ukraine two Patriot air defences and has promised a third.
The two arrests in Bavaria follow several other high-profile espionage cases in Germany.
Last summer a German national working for the military procurement agency was arrested in the western city of Koblenz on suspicion of handing information to Russian diplomats in Bonn and Berlin.
In a separate scandal, the former boss of a collapsed German payment processing company, Wirecard, is believed to have fled to Moscow after the firm collapsed. Jan Marsalek is now suspected of being a Russian spy, who recruited officials in Austria to pass on sensitive information.
- This story was first published by the BBC