German Chancellor Angela Merkel arrives on crutches for the weekly cabinet meeting in Berlin January 29, 2014. Merkel suffered an injury while skiing during her holidays in December 2013. [Photo/Agencies]

Snowden's first revelations last June caused indignation in Germany, which is especially sensitive about surveillance after its experience of abuse by the Gestapo under the Nazis and by the Stasi in Communist East Germany during the Cold War.

Reports that the NSA even monitored Merkel's mobile phone added to the anger in Germany, which has pushed - in vain so far - for a 'no-spy' agreement with the United States.

Snowden told German TV on Sunday that the NSA also spied on German industry, such as engineering firm Siemens. He has claimed asylum in Russia but has offered to go to Berlin to help a Bundestag probe into NSA activities.

Merkel said it was not helpful to link the NSA row to talks between Washington and the European Union about a transatlantic free trade area, adding that there was no other "leverage" the EU could use against the Americans regarding espionage other than "the strength of our arguments".

"Millions of people who live in undemocratic countries are today looking closely at how the democratic world reacts to security threats," Merkel said, adding that, for all their differences, "Germany can wish for no better partner than the United States".