The American envoy "was asked to help in the swift clarification" of the case, the foreign ministry said on Friday.

German officials confirmed the arrest but released no other details.

US-German ties were strained after accusations last year that the US National Security Agency bugged Chancellor Angela Merkel's phone as part of a huge surveillance program.

The scale of the agency's global spy program was revealed in documents leaked by a former intelligence contractor, Edward Snowden.

The revelations also raised feeling in Germany against American surveillance.

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 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.

NTJ/MB