"This is not a situation in which we are rifling through ordinary emails" of huge numbers of citizens in the United States or elsewhere, the president declared during a news conference with German Chancellor Angela Merkel. He called it as a "circumscribed, narrow" surveillance programme.
"Lives have been saved," Obama said, adding that the programme has been closely supervised by the courts to ensure that any encroachment of privacy is strictly limited.
"There has to be proportionality," she said. She added that their discussion on the matter Wednesday was "an important first step" over striking a balance.
The two leaders spoke to the media after meeting privately on a range of issues confronting US and European leaders, including the fragile effort to bring peace in Afghanistan, where peace talks with the Taliban are in the offing to find ways to end the nearly 12-year war.
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Earlier today, Afghan President Hamid Karzai suspended talks with the US on a new security deal to protest the way his government was being left out of the initial peace negotiations with the Taliban.
But he said it was important to pursue a parallel track toward reconciliation even as the fighting continues, and it would up to the Afghan people whether that effort ultimately bears fruit.