German federal prosecutors said on Wednesday that a German national working for the military had been arrested on suspicion of spying for Russia, amid warnings of increased espionage activity by Moscow.

"The accused is strongly suspected of working for a foreign intelligence service," the federal prosecutor's office said in a statement, adding that the suspect had offered his services to "the Russian embassy in Berlin" among other Russian institutions.

The arrested man, identified only as Thomas H. in line with German privacy rules, was detained on July 27 in the western city of Koblenz and his apartment and workplace were searched, prosecutors said. He was remanded in custody on Wednesday.

Thomas H. had worked for the army's Equipment, Information Technology and In-Service Support Department.

"In May 2023 he approached the Russian general consulate in Bonn and the Russian embassy in Berlin and offered his cooperation," prosecutors said.

"In the process, he passed on information he had obtained in the course of his professional activities for them to be passed on to a Russian intelligence service."

The investigation against him was conducted in close coordination with military intelligence and the domestic security agency, the BfV.

The arrest came after the BfV in June warned against the risk of an "aggressive Russian espionage operation" as Moscow wages its full-scale invasion of Ukraine.

Western sanctions against Russia and their support for Ukraine's military efforts meant the Kremlin had an "increased interest" in information gathering, the BfV said in its annual report.

"In future, a more clandestine and aggressive Russian espionage operation is to be expected as well as cyberspace activities originating from Russia," the BfV said.

Russian intelligence services were trying to "bring new employees to Germany", as well as pursuing or renewing activities with existing staff.

In mid-April, Berlin expelled a number of Russian diplomats over espionage concerns, prompting the tit-for-tat expulsion of 20 German diplomats from Moscow.

A month later, Russia put a limit of 350 on the number of German personnel allowed in Russia, in effect forcing hundreds of civil servants and local employees working for German institutions in Russia to leave the country.

Berlin swiftly retaliated, ordering four of Moscow's five consulates in Germany to close.