The director of the National Security Agency, under fire for the widespread surveillance of U.S. phones and Internet servers, told lawmakers Wednesday that such data collections have foiled "dozens" of terrorist plots.

Addressing the simmering controversy in the first open congressional hearing on the matter, Army Gen. Keith Alexander, NSA's director and head of U.S. Cyber Command, defended the collection of phone records and online data of millions of Americans, saying such practices were essential to protecting the homeland.

But while Alexander credited the programs with preventing dozens of attacks, he told the Senate Appropriations Committee that he didn't have an exact count. He said he would provide a number by week's end.

The Obama administration continues to defend the top-secret snooping, renewing the post-9/11 debate about the trade-offs between privacy and security.

"They do this lawfully. They take compliance, oversight, protecting civil liberties and privacy and the security of this nation to their heart every day," Alexander said of the NSA. "I could not be more proud of the men and women of NSA and Cyber Command."

Lawmakers also focused on Edward Snowden, the former CIA employee and NSA contractor who leaked information about the classified programs -- and who has kick-started a second debate about the vast number of private employees being given access to sensitive government secrets.

Sen. Dick Durbin, D-Ill., noted Snowden's "limited education and work experience" and asked if Alexander was troubled by his access to classified information.

"I am," Alexander replied.

Snowden is believed to be hiding in Hong Kong, where he is hoping to avoid extradition back to the U.S. The Justice Department is investigating Snowden but has yet to formally file charges against him.

Alexander dismissed Snowden's claim that the NSA could tap into any American's phone or Internet records without getting approval from a court.

"I know of no way to do that," he said.

Intelligence officials have provided lawmakers with closed-door briefings on the NSA programs throughout the week, trying to fend off charges that they left Congress in the dark about the controversial programs.

"I think it's been amply demonstrated that with regards to both sections of the Patriot Act and the programs that exist under those authorities that members of Congress were briefed or had the opportunity to be briefed on them," White House press secretary Jay Carney said. "It is certainly the case that some members of Congress did not avail themselves of the opportunity to be briefed, but that's certainly their prerogative."

Civil libertarians and Obama's own liberal base have united in pressing the White House for more details about the surveillance operations and their targets. The president has declined to give a detailed accounting of the tactics, answering two questions in a press conference last week and leaving fuller explanations to his surrogates.

Despite the uproar in Washington, 53 percent of Americans think the government's collection of phone records remains critical to combating terrorism, a new CBS poll showed.