German authorities are investigating a second spy case reportedly involving the US, a week after the arrest of a German intelligence employee cast a new shadow over relations between the two countries.
The new case, reported on Wednesday, was believed to be more serious than the first, the Sueddeutsche Zeitung daily said.
Federal prosecutors said police raided properties in the Berlin area on "initial suspicion of activity for an intelligence agency".
They did not elaborate or specify what intelligence agency was involved, but said they had not made an arrest.
"We have made investigations in two cases of suspected espionage - a very serious suspicion," government spokesman Steffen Seibert said. He declined to provide further details.
Sueddeutsche Zeitung reported, without naming sources, that the man being investigated worked at Germany's defence ministry and was suspected of spying for the US.
Die Welt newspaper reported, also without naming sources, the man was a soldier in the German army who had aroused the suspicion of the country's military counter-intelligence agency because of his close contacts to alleged US spies.
Defence ministry spokesman Lieutenant Colonel Uwe Roth declined to confirm the reports, but said the case fell "into the ministry's area of responsibility" and that Defence Minister Ursula von der Leyen was informed.
US State Department officials travelling with Secretary of State John Kerry in Beijing had no immediate comment.
Last week, a 31-year-old German intelligence employee was arrested on suspicion of spying for foreign powers since 2012.
German media have reported that he spied for the US and came to authorities' attention when he offered his services to Russian officials in Germany by e-mail.
The case has frayed relations between Berlin and Washington, which were already strained by reports last year that the US National Security Agency eavesdropped on Chancellor Angela Merkel's cellphone.
Merkel said on Monday that if it proved to be true that the officer was spying for Nato ally the US, it would be a "serious case".
The US ambassador to Berlin was summoned to the foreign ministry last Friday and was asked to clarify the case.
Ambassador John Emerson was at the ministry again on Tuesday for a meeting with a senior official, foreign ministry spokesman Martin Schaefer said.
US embassy spokesman Peter Claussen said the meeting was arranged at the embassy's request.
Additional reporting by Reuters