German chancellor Angela Merkel is pointing fingers at the U.S. REUTERS/Fabrizio Bensch German authorities have arrested a German man employed by the domestic intelligence agency (BND) 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.

The affair risks further straining ties with Washington which were damaged by revelations last year of mass surveillance of German citizens by the U.S. National Security Agency, which included monitoring Chancellor Angela Merkel's mobile phone.

The man has admitted passing to an American contact details about a special German parliamentary committee set up to investigate the spying revelations made by former U.S. 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 German intelligence.

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.

The United States embassy in Berlin declined to comment.

Germany is particularly sensitive about surveillance because of abuses by the East German Stasi secret police and the Nazis. Berlin has demanded that Washington agree to a "no-spy" with its close ally, but the United States has been unwilling.

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.($1 = 0.7331 Euros)

(Reporting by Thorsten Severin; Writing by Alexandra Hudson; Editing by Stephen Brown and Hugh Lawson)