BERLIN - "Not everything that is technically possible is politically justified," said German Foreign Minister Guido Westerwelle Sunday in reference to the recent revelations the National Security Agency has been listening in on German Chancellor Angela Merkel's cell phone for over a decade.

"Wiretapping between friends and allies is, on the political level, a very damaging action. It threatens to undo the bonds that hold us together, bonds that are vital – today more than ever before – if we are to succeed in shaping a shared future in the 21st century."

Over the weekend, reports surfaced that Merkel's cellphone was being monitored by U.S. personnel working from within the American embassy in Berlin, which is located some 800 meters from the German chancellor's office.

Germany's outrage over the NSA eavesdropping prompted it to summon the U.S. ambassador on Thursday for the first time in living memory - an unprecedented post-war diplomatic rift.

Earlier in the week, an apologetic U.S. President Barack Obama told Merkel he was unaware American intelligence services had tapped her phone, German newspapers The Algemeiner and Der Spiegel reported.

Merkel's government says she complained to Obama on Wednesday after receiving information that her cell phone may have been monitored, and "made clear that she unequivocally condemns and regards such practices as fully unacceptable," German government spokesman Steffen Seibert said.