"Dozens" of terror attacks had been thwarted by a program to gather and analyze massive amounts of Internet and phone data, the National Security Agency chief told US lawmakers Wednesday.

Facing skeptical questions from lawmakers after a rogue technician leaked the secret operation, NSA chief General Keith Alexander, who also heads the US Cyber Command, insisted that the program is legal and operates under judicial oversight.

"It's classified but it's dozens of terrorist events that these have helped prevent," he told the hearing, the first time he had been questioned in public since 29-year-old former contractor Edward Snowden spilled the beans.

"I want the American people to know that we're being transparent in here," he insisted, warning that "the trust of the American people" was a "sacred requirement" if his agency was to be able to do its job.

Could the revelation help terrorists avoid surveillance? "They will get through, and Americans will die," Alexander said. "Great harm has already been done by opening this up. The consequence I believe is our security has been jeopardized."

Snowden, a technician working for a private contractor and assigned to an NSA base in Hawaii, disappeared last month after downloading a cache of secret documents and surfaced over the weekend in Hong Kong to give media interviews.

China has said little on the case, and on Thursday appeared to keep its distance.

"I have no information to offer," Chinese foreign ministry spokeswoman Hua Chunying told reporters in Beijing when asked about Snowden, who is hiding in China's semi-autonomous southern territory.

Hua dodged questions about whether Washington had approached Beijing seeking Snowden's extradition, and how China would react if Snowden applied for asylum.

Snowden told Hong Kong daily the South China Morning Post that he would resist any attempt at extradition.

Chinese state media has also remained relatively quiet on the case, but the China Daily said Thursday that news of the US program "is certain to stain Washington's overseas image and test developing Sino-US ties."

"How the case is handled could pose a challenge to the burgeoning goodwill between Beijing and Washington given that Snowden is in Chinese territory and the Sino-US relationship is constantly soured on cybersecurity," the government-owned newspaper said.

Snowden embarrassed and infuriated President Barack Obama's administration by revealing that the NSA had gathered call log records for millions of US phone subscribers and targeted the Internet data of foreign Web users.

While some hail Snowden as a whistleblower who carried out an act of civil disobedience to expose US government overreach, others -- including US spy chiefs and some senior lawmakers -- say he is a traitor.

Alexander said he had "grave concerns" about how Snowden was able to gain access to critical classified information with a limited education and little work experience.

Several investigations are underway and Snowden may yet face criminal charges, but in the meantime, debate is raging about the legality and utility of the NSA's broad-brush approach to sweeping up private data.

"How do we get from reasonable grounds... to all phone records, all the time, all locations?" asked Senator Jeff Merkley, a supporter of limiting government surveillance powers.

Holding up his own phone, he asked Alexander: "What authority gave you the grounds for acquiring my cell phone data?"

The general repeated the administration's defense that, while the NSA did gather large quantities of telephone metadata, it could not mine the logs to target a specific user without authorization from a secret court.

"We do not see a trade-off between security and liberty," he said, insisting that the NSA and US Cyber Command are "deeply committed to compliance with the law and the protection of privacy rights."

Alexander however said he welcomed the "debate" sparked by the leaks.

Senator Mark Udall expressed skepticism at this nod to openness.

"It's very, very difficult, I think, to have a transparent debate about a secret program written by a secret court, issuing secret court orders based on secret interpretations of the law," he said.

In Hong Kong, Snowden was unrepentant.

"I'm neither traitor nor hero. I'm an American," he said.

Snowden told the South China Morning Post there had been more than 61,000 NSA hacking operations globally, targeting powerful "network backbones" that can yield access to hundreds of thousands of individual computers.

There were hundreds of targets in mainland China and Hong Kong, he was quoted as saying, alleging this exposed "the hypocrisy of the US government when it claims that it does not target civilian infrastructure."

Snowden accused the US government of "trying to bully" Hong Kong into expelling him before he can reveal details of alleged NSA snooping on communications inside the Chinese financial and trading hub.

He also pledged to resist any extradition attempt.

"I have had many opportunities to flee HK, but I would rather stay and fight the US government in the courts, because I have faith in HK's rule of law," he said, according to the paper.

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