"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 program.
"Lives have been saved," Obama said, adding that the program has been closely supervised by the courts to ensure that any encroachment of privacy is strictly limited.
Merkel, for her part, said it was important to continue debate about how to strike "an equitable balance" between providing security and protecting personal freedoms.
"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.
Merkel appeared to be looking to avoid a public rift with Washington over the surveillance program, particularly since Germans benefit from U.S. intelligence. Much of the German criticism of the program has come from her junior coalition partners, facing the prospect of losses in the September election and looking for an issue.
The two leaders spoke to the media after meeting privately on a range of issues confronting U.S. 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. Earlier Wednesday, Afghan President Hamid Karzai suspended talks with the United States on a new security deal to protest the way his government was being left out of the initial peace negotiations with the Taliban.
Obama said the U.S. had anticipated "there were going to be some areas of friction, to put it mildly, in getting this thing off the ground. That's not surprising. They've been fighting there for a long time" and mistrust is rampant.
Karzai said Wednesday that peace talks cannot begin amid "fighting and bloodshed." But Obama 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.
On another world trouble spot, the 2-year-old Syrian civil war, the president declined to provide details on the type of military support the U.S. will provide to opposition forces. But he said the administration had been consistent in working toward the over-riding goal of a Syria that is "peaceful, non-sectarian, democratic, legitimate, tolerant."
"I cannot and will not comment on specifics around our programs related to the Syrian opposition," he said.
The president said while world leaders at the just-completed Group of 8 summit in Northern Ireland could not agree on whether Syrian President Bashar Assad must go, he believes Assad cannot regain legitimacy.
And the president offered reassurances on another issue of particular concern in Germany. In response to a question from a German reporter, Obama said the United States doesn't use Germany as a launching point for unmanned drones to strike terrorist targets. He said he knows there have been some reports in Germany speculating that was the case, but it's not so.
Later Wednesday, Obama planned to draw attention to his plan for a one-third reduction in U.S. and Russian arsenals, rekindling a goal that was a centerpiece of his early first-term national security agenda.
His 26-hour whirlwind visit to the German capital caps three days of international summitry for the president and marks his return to a place where he once summoned a throng of 200,000 to share his ambitious vision for American leadership.
Obama will make the case for his nuclear plan during a speech at Berlin's iconic Brandenburg Gate. His address comes nearly 50 years after John F. Kennedy's famous Cold War speech in this once-divided city, and five years after Obama spoke in the city during his 2008 run for president.
The president has previously called for reductions to the stockpiles and is not expected to outline a timeline for this renewed push. But by addressing the issue in a major foreign policy speech, Obama is signaling a desire to rekindle an issue that was a centerpiece of his early first-term national security agenda.
Five years later, Obama comes to deliver a highly anticipated speech to a country that's a bit more sober about his aspirations and the extent of his successes, yet still eager to receive his attention at a time that many here feel that Europe, and Germany in particular, are no longer U.S. priorities. A Pew Research Center poll of Germans found that while their views of the U.S. have slipped since Obama's first year in office, he has managed to retain his popularity, with 88 percent of those surveyed approving of his foreign policies.
Obama also has an arc of history to fulfill.
Fifty years ago next week, President Kennedy addressed a crowd of 450,000 in that then-divided city to repudiate communism and famously declare "Ich bin ein Berliner," German for "I am a Berliner." Since then, presidents from Ronald Reagan to Bill Clinton have used Berlin speeches to articulate broad themes about freedom and international alliances.
Obama, fresh from a two-day summit of the Group of Eight industrial economies, placed his hand over his heart outside the sunny presidential palace as a German military band played "The Star-Spangled Banner," the American national anthem. He and German President Joachim Gauck inspected a lineup of German military troops before entering the palace, stopping to greet children who waved American and German flags.
The visit was attracting widespread attention in Germany. People waved and snapped photos as Obama sped by after his arrival and a thick cluster awaited the motorcade as it passed the Brandenburg Gate. An evening news show in Berlin devoted itself to the president's visit, highlighting "Das Biest," or "The Beast," as the president's armored limousine is called.
There have been a few small protests, including one directed against the National Security Agency's surveillance of foreign communications, where about 50 people waved placards taunting, "Yes, we scan."
Merkel has said she was surprised at the scope of the spying that was revealed and said the U.S. must clarify what information is monitored. But she also said U.S. intelligence was key to foiling a large-scale terror plot and acknowledged her country is "dependent" on cooperating with American spy services.
For Merkel, the visit presents an opportunity to bolster her domestic standing ahead of a general election in September.
The U.S. and the Germans have clashed on economic issues, with Obama pressing for Europe to prime the economy with government stimulus measures, while Merkel has insisted on pressing debt-ridden countries to stabilize their fiscal situations first.
But the two sides have found common ground on a trans-Atlantic trade pact between the European Union and the U.S. At the just-completed G-8 summit, the leaders agreed to hold the first talks next month in the U.S.
Associated Press writers Julie Pace, Robert Reid and Frank Jordans contributed to this report.