The German Foreign Ministry has summoned the United States ambassador in Berlin after an officer in the German secret service was arrested on suspicion of spying for the US.
The German Federal Prosecutor's office said in a statement that a 31-year-old man had been arrested on suspicion of being a foreign spy, but it gave no further details.
The affair risks further straining ties with Washington which were damaged by revelations last year of mass surveillance of German citizens by the US National Security Agency,
The surveillance included monitoring Chancellor Angela Merkel's mobile phone.
The man has admitted passing to a US contact details about a special German parliamentary committee set up to investigate the spying revelations made by former US intelligence contractor Edward Snowden, politicians said.
The parliamentary committee investigating the NSA affair holds some confidential meetings.
"This was a man who had no direct contact with the investigative committee... He was not a top agent," said one politician.
The suspect had offered his services to the United States voluntarily, a source said.
The United States embassy in Berlin declined to comment.
Bild newspaper said said that the man had worked for two years as a double agent and had stolen 218 confidential documents.
He sold the documents, three of which related to the work of the committee in the Bundestag, for €25,000, Bild said, citing security sources.