The German authorities have summoned the US ambassador in Berlin after a man was arrested on suspicion of spying.
The US diplomat "was asked to help in the swift clarification" of the case, the foreign ministry said.
German officials confirmed the arrest but released no other details.
US-German ties were strained after allegations last year that the US National Security Agency bugged Chancellor Angela Merkel's phone as part of a huge surveillance programme.
The scale of the agency's global spy programme was revealed in documents leaked by a former intelligence contractor, Edward Snowden.
The revelations also 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 intelligence service, the BND or Bundesnachrichtendienst.
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.
The committee co-chair, Social Democrat MP Christian Flisek, said if the suspicion of a targeted attack on a German constitutional body was confirmed "that would set the level of trust back to zero and result in political consequences," the Wall Street Journal reported.
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