On Saturday, Spiegel magazine reported that the NSA's Special Collection Service (SCS) had listed Merkel's mobile telephone since 2002, beginning under the George W. Bush administration, and that it had remained on the list weeks before Obama visited Berlin in June.

According to the Frankfurter Allgemeine Sonntagszeitung, Obama had told Merkel during a phone conversation on Wednesday that he had not known of the bugging. However, a report in Bild am Sonntag published Sunday cites an unnamed NSA official who said that the US leader instead ordered the program be escalated.

The newspaper reports that Obama knew that the NSA had been spying on Merkel's mobile phone since at least 2010, when NSA chief Keith Alexander personally informed him of the operation.

Messages, calls accessed

In addition to Merkel's mobile phone provided by her conservative political party, the NSA also listened in on a supposedly secure phone that Merkel received during the summer, according to Bild am Sonntag.

Only a special, secure landline phone in her office was reportedly not accessible to electronic tapping.

The NSA's findings, including the contents of SMS messages and phone calls, were reported directly to the White House and evidence indicates the operation continued until the "immediate past", according to Bild am Sonntag.

Bild am Sonntag also reported that the NSA eavesdropped on Merkel's predecessor, Gerhard Schröder, after then-President George W. Bush launched a spying program in 2002. The newspaper suggested a key reason for the operation was Schröder's refusal to support the Iraq War, and it was simply extended after Merkel took office in 2005.

Embassy listening post

Spiegel's earlier report had also said that NSA and CIA staff had tapped government communications with high-tech surveillance from the US Embassy in Berlin.

Spiegel cited a Special Collection Service document saying the agency had a "not legally registered spying branch" in the Berlin embassy, the exposure of which would lead to "grave damage for the relations of the United States to another government."

Quoting a secret 2010 document, Spiegel reported that such branches existed in about 80 locations worldwide, including Paris, Madrid, Rome, Prague, Geneva and Frankfurt. The magazine reported that it remained unclear whether the SCS had recorded conversations or just connection data. On Friday, Germany announced that experts considered Merkel's state-related calls safe given that she normally uses encrypted phones.

US-German tension

Germany summoned the US ambassador, John B. Emerson, this week to express concerns over US spying actvities. The dispute over US surveillance first emerged earlier this year after reports that Washington had bugged European Union offices and had tapped half a billion phone calls, emails and text messages in Germany in a typical month.

In August, Merkel's government announced weeks before parliamentary elections that the US had given assurances that spies had upheld German law. Germany will send intelligence chiefs to Washington next week to seek answers to the allegations.

Meanwhile, several hundred people gathered in Washington protest US surveillance and call for a legislation limiting the NSA's snooping capabilities. Demonstrators held signs reading "Thank You Edward Snowden!" and "Stop Mass Spying" as they marched near the US Capitol building.

Disclosures from former intelligence contractor turned whistle-blower Edward Snowden published internationally have prompted Brazil and Germany to begin drafting a UN General Assembly resolution. The vote would demand that excessive spying and invasions of privacy be ended following complaints from world leaders such as Brazilian President Dilma Rousseff over alleged NSA tapping of their communications.

dr,mgk/ch (dpa, AFP, Reuters, AP)