An employee of Germany's intelligence service has been arrested on suspicion of spying for the US, reports say.
The man is said to have been trying to gather details about a German parliamentary committee that is investigating claims of US espionage.
German authorities have asked the US ambassador for "swift clarification".
The US National Security Agency (NSA) was last year accused of bugging the phone of Chancellor Angela Merkel as part of a huge surveillance programme.
The scale of the agency's global spy programme was revealed in documents leaked last year by a former intelligence contractor, Edward Snowden.
The revelations about the NSA put a strain on ties between Germany and the US and raised feeling in Germany against American surveillance.
The BBC's Steve Evans in Berlin says the new allegation of American spying on an ally may make it harder for the US to get German help in its efforts to oppose Russian activity in Ukraine, and also to control Iranian nuclear ambitions.
German media say the man arrested this week is a 31-year-old employee of the federal service, the BND or Bundesnachrichtendienst.
The German federal prosecutor's office confirmed the man's arrest, but gave no other details.
A spokesman for Ms Merkel said she had been informed of the arrest, as had the members of the nine-strong parliamentary committee investigating the activities of foreign intelligence agencies in Germany.
"The matter is serious, it is clear," spokesman Steffen Seibert told the Frankfurter Allgemeine newspaper,
Der Spiegel news magazine said the man was believed to have passed secret documents to a US contact in exchange for money.
However, one unnamed politician told Reuters news agency the suspect had offered his services to the US voluntarily.
"This was a man who had no direct contact with the investigative committee... He was not a top agent," the source said.
Germany is particularly sensitive to reports of espionage on its territory because many of its citizens from the formerly communist east of the country were spied upon by the Stasi secret police.