Speaking from California where was to meet with Chinese Premier Xi Jinping, Obama said Friday there must be a tradeoff between personal privacy and national security.

"We have to make choices as a society," he told reporters. "It's important to recognize that you can't have 100 percent security and also then have 100 percent privacy and zero inconvenience."

He added an assurance to Americans that "nobody is listening in on your telephone calls."

Far-reaching surveillance programs

On Wednesday the Guardian newspaper revealed that the National Security Agency (NSA) has been collecting the phone records of tens of millions of US phone customers. A leaked document obtained by the Guardian gave the government authority to collect information on Verizon's customers, but intelligence experts have said the program extends to other companies as well.

A separate program called PRISM revealed by the Guardian and The Washington Post newspaper Thursday allows authorities to access the central servers of companies like Google, Microsoft, Facebook and Apple. The government can then obtain emails, photos, video chats and other files, allowing a person's movements and contacts to be tracked.

The president insisted Friday that PRISM does not apply to US citizens or foreign residents, however.

"They help us prevent terrorist attacks," Obama said of the programs, adding that they were worth the "modest encroachments on privacy."

"They make a difference in our capacity to anticipate and prevent possible terrorist activity," he said.

Obama stressed the programs were overseen by federal judges and Congress and said his administration had also instituted audits to ensure safeguards were observed.

The president's comments Friday were his first since the programs were publicly revealed.

Civil liberties debate

The revelations about the surveillance programs have fueled a domestic debate on civil liberties and national security. Democratic Senator Ron Wyden called for a debate "in the Congress and in the country" that he said was "long overdue." Last year Wyden, the Senate Intelligence Committee member, wrote to Attorney General Eric Holder warning of the dangers of over-reaching data-gathering practices.

Congressman Jim Sensenbrenner, who authored the Bush-era Patriot Act in 2011 that is the basis for many domestic surveillance policies, said he was "extremely troubled" by the programs revealed in the Guardian and The Washington Post.

"While I believe the Patriot Act appropriately balanced national security concerns and civil rights, I have always worried about potential abuses," he said in a statement. "Seizing the phone records of millions of innocent people is excessive and un-American."

Obama said he welcomed the public debate over government surveillance, but added the programs had been classified for national security reasons.

"In the abstract, you can complain about 'Big Brother' and how this is a potential program run amok, but when you actually look at the details, then I think we've struck the right balance," he said.

dr/rc (AP, AFP, Reuters)