The United States has launched a criminal investigation and is taking "all necessary steps" to prosecute Edward Snowden for exposing secret US surveillance programs, the FBI director said Thursday.

Robert Mueller, who is to step down soon after more than a decade leading the Federal Bureau of Investigation, defended the Internet and phone sweeps as vital tools that could have prevented the attacks of September 11, 2001.

Snowden's disclosures "have caused significant harm to our nation and to our safety," Mueller told lawmakers at a House Judiciary Committee hearing.

As to Snowden, "he is the subject of an ongoing criminal investigation," Mueller said. "We are taking all necessary steps to hold the person responsible for these disclosures."

Mueller's comments confirm that the US government is pursuing Snowden, the 29-year-old American IT specialist who has admitted to leaking information about far-reaching surveillance programs.

Snowden, who worked as a subcontractor handling computer networks for the National Security Agency (NSA), is in Hong Kong, a semi-autonomous Chinese territory, where he has vowed to contest any possible extradition in court.

Mueller defended the collection of US phone records and Internet data related to foreign targets, which officials maintain are legal programs approved by federal judges and in accordance with the Constitution.

"The program is set up for a very limited purpose and a limited objective, and that is to identify individuals in the United States who are using a telephone for terrorist activities and to draw that network," he said.

Mueller told lawmakers that one of the 9/11 hijackers, Khalid al-Mihdhar, had called a known Al-Qaeda safe house in Yemen from the US city of San Diego.

"If we had had this program in place at the time, we would have been able to identify that particular telephone number in San Diego," Mueller said.

"If we had the telephone number from Yemen we would have matched up to that telephone number in San Diego, got further legal process, identified al-Mihdhar."

Many lawmakers remained skeptical. "It's my fear we are on the verge of becoming a surveillance state," Democrat John Conyers said, alarmed at the scale and secrecy of the surveillance programs.

General Keith Alexander, the head of the National Security Agency, told lawmakers on Wednesday that "dozens" of terror attacks had been thwarted by programs, and that the leaks had caused "great harm" to national security.

Senator Dianne Feinstein, the chair of the Senate intelligence committee, said that the NSA on Monday will release the specific number of attacks prevented.

Some lawmakers opposed to the domestic surveillance techniques have demanded proof that the data collection yields results.

Snowden, a technician working for a private contractor and assigned to an NSA base in Hawaii, surfaced over the weekend in Hong Kong to give media interviews.

In addition to disclosing the NSA's acquisition of phone logs and data from nine Internet giants -- including Google, Microsoft and Facebook -- Snowden also described secret global hacking operations.

On Friday, the South China Morning Post reported that Snowden has classified US documents showing the machines the NSA has targeted in China and Hong Kong.

Snowden showed the newspaper a small sample of the records, which detailed the Internet Protocol (IP) addresses of targets and dates of cyber attacks.

IP addresses are unique numbers assigned to individual computers and other devices attached to the Internet. The SCMP did not publish the addresses or identify individuals who might have been targeted, or was able to confirm the authenticity of the information.

The documents however also showed whether a cyber attack was ongoing, and appeared to suggest a hacking success rate of 75 percent.

"I don't know what specific information they were looking for on these machines, only that using technical exploits to gain unauthorized access to civilian machines is a violation of law. It's ethically dubious," Snowden told the newspaper.

Earlier, Snowden told the Morning Post that 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.

The US administration has said 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.

US officials have also said the Internet monitoring program did not target Americans or even foreigners on US soil.

China has said little about the case, and foreign ministry spokeswoman Hua Chunying on Thursday dodged questions about whether Washington had sought Snowden's extradition and how China would react if he applied for asylum.

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

US officials have accused China of state-sponsored hacking targeting the military, infrastructure and corporations, charges denied by Beijing, which insists China is itself the target of considerable foreign cyberattacks.

burs-ddl/ch