The National Security Agency (NSA) snooping muddle seems to grow murkier by the day. Causing further embarrassment to the Obama administration, new revelations say that U.S. may have been listening into German Chancellor Angela Merkel's phone calls for over a decade. Reports said President Barack Obama apologised to the German leader and told her he would have ended it had he known about it.

NSA spying has put both Germany and U.S. in a first of its kind post-war diplomatic stand-off. The Germans made their outrage known by taking an unprecedented action of summoning the U.S. ambassador to lodge its protest and demand answers.

German publication, Der Spiegel reported that Ms Merkel's mobile phone was listed on NSA's Special Collection Service (SCS) since 2002. Marked as "GE Chancellor Merkel," the report said it remained there even weeks before President Obama's visit to Berlin in June 2013.

The report refers to a "not legally registered spying branch" set-up in the U.S. Embassy in Berlin, the exposure of which the SCS document quoted by the Der Spiegel said could pose a "grave damage for the relations of the United States to another government". It was revealed that NSA and CIA staff stationed at the branch in the Embassy tapped German government communication with high-tech surveillance.

The publication further quoted the secret documentation saying that similar branches were set-up in about 80 locations across the world. These sites included Paris, Madrid, Rome, Prague, Geneva and Frankfurt, the report said.

Replying to the German leader's call, on Wednesday, seeking clarification, Der Spiegel said, President Obama apologised to her and said he had not known of the bugging.

Both governments declined to make official comments to the media.

"We're not going to comment on the details of our diplomatic discussions," Caitlin Hayden, a spokeswoman for the National Security Council at the White House, was reported as saying.

German, intelligence chiefs will be visiting Washington this week to seek further answers to Chancellor Merkel's phone tapping allegations.

The US-Europe diplomatic rift began after news emerged that NSA had bugged the European Union office and several European governments, to listen in to phone conversations, and read emails and text messages.

Since, former NSA contractor Edward Snowden leaked NSA spying documents raised international alarm; President Obama has ordered review of the entire surveillance program.