In diplomatic terms, the invitation was a step below last month's summoning of the U.S. ambassador in Berlin following allegations that American intelligence may have targeted Chancellor Angela Merkel's cellphone.
Government officials reacted with outrage to the report, published by German news weekly Der Spiegel and citing documents obtained through NSA leaker Edward Snowden.
German officials have since been dispatched to Washington to find out exactly what surveillance the U.S. conducted in Germany and to try to forge a "no spy" agreement between the two allied nations.
A spokeswoman for Germany's domestic intelligence agency said Tuesday that a group it set up after the first NSA spying allegations emerged in the summer is looking at the activities of British as well as U.S. intelligence. She did not comment on any results, which go to a parliamentary committee.
She spoke on condition of anonymity because the rules of her job did not allow her to be named.