China said Thursday it "has no information to offer" about a former US government subcontractor who exposed massive US phone and Internet spying and has taken refuge in Hong Kong.

The bombshell revelations by Edward Snowden, made in the semi-autonomous southern Chinese city, come amid tensions between Washington and Beijing about online espionage.

Snowden has vowed to resist any US attempt to extradite him from Hong Kong, a former British colony with an independent judiciary and a strong tradition of free speech.

Hong Kong is a special administrative region of China, meaning that Beijing is ultimately responsible for its defence and security. That relationship has drawn intense interest over what role Chinese authorities may play as the drama unfolds.

China's government and official media had remained relatively quiet during a three-day public holiday that lasted through Wednesday.

On Thursday the foreign ministry provided little insight into Beijing's thinking.

"I have no information to offer," ministry spokeswoman Hua Chunying told a regular briefing.

She was responding to a question about whether the US had approached China about Snowden's extradition, and what Beijing's reaction would be to such a request.

Hua was later asked if any extradition decision would be taken by the Chinese government or by Hong Kong's administration. She replied she had no information.

When asked about Snowden's claims to Hong Kong's South China Morning Post that the US has hacked computers in China, Hua reiterated Beijing's position that it is a major victim of cyber-attacks.

"We have repeatedly said that cyber-security is a global issue. Like other countries China also faces severe threats of cyber-attacks," she said.

"We are opposed to all forms of hacker and cyber-attacks.

"We also think that adopting double standards is not beneficial to an appropriate resolution" of the issue, she said.

Hua also repeated China's position that the international community should hold dialogue on how to "maintain peace, security, openness and cooperation in cyber-space".

China was willing to hold dialogue with the US on the matter as well, she said.

Cyber-attacks were one of several issues discussed when the two countries' presidents Barack Obama and Xi Jinping held a two-day summit in California last week.

Washington has in recent months intensified its public accusations of Chinese state-backed cyber-attacks -- an allegation which Beijing vehemently denies.

Leaks and reports recently revealed that the NSA is tapping the servers of nine Internet giants including Apple, Facebook and Google, and collecting a vast sweep of phone records.

The revelations have triggered major debate about privacy and security.

Supporters of Snowden call him a brave whistleblower, while the White House has said the international dragnet is needed to keep Americans safe from terror.

The China Daily on Thursday cited an analyst who noted the irony that the US's surveillance programme was exposed just as it began ramping up pressure on Beijing.

"It turns out that the biggest threat to the pursuit of individual freedom and privacy in the US is the unbridled power of the government," the paper quoted China Foreign Affairs University researcher Li Haidong as saying.

The programme "is certain to stain Washington's overseas image and test developing Sino-US ties", it said, citing experts.

"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 cyber-security."