An apologetic U.S. President Barack Obama told German Chancellor Angela Merkel he was unaware American intelligence services had tapped her phone, German newspapers The Algemeiner and Der Spiegel reported over the weekend.

According to the latter, Obama told the German leader that had he known about the surveillance, he would have ordered an immediate end to it.

Germany's outrage over reports that the National Security Agency had bugged Merkel's phone prompted it to summon the U.S. ambassador this week for the first time in living memory, an unprecedented post-war diplomatic rift.

Der Spiegel said Merkel's cell phone had been listed by the NSA's Special Collection Service (SCS) since 2002 - marked as "GE Chancellor Merkel" - and was still on the list weeks before Obama visited Berlin in June.

In an SCS document cited by the magazine, the agency said it had a "not legally registered spying branch" in the U.S. embassy in Berlin, the exposure of which would lead to "grave damage for the relations of the United States to another government".

From there, NSA and CIA staff were tapping communication in the Berlin's government district with high tech surveillance.

Quoting a secret document from 2010, Der Spiegel said such branches existed in about 80 locations around the world, including Paris, Madrid, Rome, Prague, Geneva and Frankfurt.

The magazine said it was not clear whether the SCS had recorded conversations or just connection data.

Earlier this week, White House spokesman Jay Carney told reporters that Obama "assured the chancellor that the United States is not monitoring and will not monitor the communications of the chancellor."

"Der Spiegel" reported that National Security Advisor Susan Rice discussed the issue with Christopher Huizingen, a senior aide to Merkel. The United States, she told him, can stop such surveillance from taking place in the present and in the future – but not in the past.

The New York Times meanwhile reported that the NSA's surveillance of the German Chancellor began over a decade ago, during George W. Bush's tenure.