An employee of Germany's BND foreign intelligence agency has been arrested on suspicion of spying for the United States, two politicians with knowledge of the affair told Reuters on Friday.
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. Investigations were continuing, it said.
The case 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, including the monitoring of Chancellor Angela Merkel's mobile phone.
The man, who is German, has admitted passing to an American contact details about a special German parliamentary committee set up to investigate the spying revelations made by former US intelligence contractor Edward Snowden, the politicians said.
Both lawmakers are members of the nine-person parliamentary control committee, whose meetings are confidential, and which is in charge of monitoring the work of German intelligence agencies.
The parliamentary committee investigating the NSA affair also 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 of the politicians, who spoke on condition of anonymity. The suspect had offered his services to the United States voluntarily, the source said.
Merkel's spokesman Steffen Seibert said: "We don't take the matter of spying for foreign intelligence agencies lightly".
When asked whether Merkel had discussed it with President Barack Obama during a phone conversation on Thursday night, he answered they had talked about foreign affairs.
The United States embassy in Berlin, the State Department in Washington and the White House all declined to comment.
Germany is particularly sensitive about surveillance because of abuses by the East German Stasi secret police and the Nazis. After the Snowden revelations, Berlin demanded that Washington agree to a "no-spy agreement" with its close ally, but the United States has been unwilling.
Germany's Sueddeutsche Zeitung newspaper and WDR and NDR broadcasters reported that the alleged spy was first detained on suspicion of contacting Russian intelligence agents. He then admitted he had worked with Americans.
Bild newspaper said in an advance copy of an article to be published on Saturday 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 euros ($34,100), Bild said, citing security sources.
Opposition lawmakers called for diplomatic consequences if the allegations should prove true.
The head of parliament's committee investigating the NSA affair, Patrick Sensburg, said its members had long feared they may be targeted by foreign intelligence agents and had taken special measures.
