German media reported that eavesdropping on Merkel's phone may have started in 2002, when she was Germany's main opposition leader and three years before she became chancellor.
The National Security Agency (NSA) stopped spying on Merkel after the White House learned of the snooping in an internal mid-year review, the Wall Street
Journal reported Monday, the first public acknowledgement that there was US eavesdropping.
The review, which the president ordered in August, showed that the NSA had tapped the phones of some 35 world leaders. The White House ended programs
tracking several of the leaders including Merkel, according to the Journal.
That report contradicts US intelligence sources which told Germany's Bild am Sonntag newspaper that NSA chief General Keith Alexander had briefed Obama on the operation against Merkel in 2010.
"Obama did not halt the operation but rather let it continue," an unnamed high-ranking NSA official told the newspaper.
Documents leaked by fugitive US defense contractor Edward Snowden showed that Merkel's phone had appeared on a list of spying targets for more than a decade, and was still under surveillance weeks before Obama visited Berlin in June, German news weekly Der Spiegel reported.
NSA spokeswoman Vanee Vines however denied the Alexander briefing claims. Alexander "did not discuss with President Obama in 2010 an alleged foreign intelligence operation involving German Chancellor Merkel, nor has he ever discussed alleged operations involving Chancellor Merkel," Vines said on Sunday.
"News reports claiming otherwise are not true," she said.
According to the Journal, Obama was "briefed on and approved of broader intelligence-collection 'priorities,'" but deputies decided on specific intelligence targets because it would have been impractical to brief the president on all of eavesdropping operations.
"These decisions are made at NSA," the unnamed official told the Journal. "The president doesn't sign off on this stuff."
However ending a surveillance program is complicated because a world leader like Merkel may be communicating with another leader that Washington is monitoring, officials told the newspaper.
"Today's world is highly interconnected, and the flow of large amounts of data is unprecedented," National Security Council spokesperson Caitlin Hayden told AFP via e-mail. "That's why the president has directed us to review our surveillance capabilities, including when it comes to our closest foreign partners and allies."
The review is looking at intelligence gathering methods "to ensure that we properly account for the security concerns of our citizens and allies," privacy concerns, "and to ensure that our intelligence resources most effectively support our foreign policy and national security objectives," she said.
The leaked Snowden documents indicate that US spy agencies accessed the electronic communications of dozens of world leaders and possibly millions of foreign nationals.
Obama should "stop apologizing"
US lawmakers on Sunday sought to play down the scandal. Representative Peter King, a Republican who chairs the House Subcommittee on Counterterrorism and Intelligence, said Obama should "stop apologizing" about the NSA phone-tapping, claiming the programs had saved "thousands" of lives.
And House Intelligence Committee Chairman Mike Rogers, a fellow Republican, told CNN: "The bigger news story here would be . . . if the United States intelligence services weren't trying to collect information that would protect US interests both (at) home and abroad."
Merkel confronted Obama over the allegations in a phone call on Wednesday, saying that if true it would be a "breach of trust".
The Frankfurter Allgemeine Zeitung saidon Sunday that Obama told Merkel in the call that he had been unaware of spying against her, while Spiegel said he
assured her that he would stop the operation at once.
Merkel's office declined to comment on what Obama said.
The White House has said it is not monitoring Merkel's phone calls and will not do so in future, but refused to say whether it did so previously.
Bild said Obama wanted to be informed about Merkel, who has played a decisive role in the eurozone debt crisis and is seen as Europe's most
As a result, the NSA stepped up its surveillance, targeting the mobile phone she uses to communicate with her conservative Christian Democratic Union (CDU)
party and her encrypted official device.
Bild said US agents monitored her conversations and her text messages, but that her secure office land line was out of reach.
According to the report, the NSA sent the intelligence gathered straight to the White House, bypassing the agency's headquarters.
Bild said Merkel's predecessor Gerhard Schröder was also in the NSA's sights because of his opposition to the US invasion of Iraq in 2003 and his
close relationship with Russian President Vladimir Putin.
Merkel's deputy spokesman Georg Streiter said Friday that "high-ranking government representatives" will soon travel to Washington to discuss the
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