China stayed silent Thursday about an American hiding out in Hong Kong after exposing a massive US surveillance operation, but state media reports said the case had tarnished Washington's image.

The revelations by former US government subcontractor Edward Snowden of Internet and phone spying -- including claims of US hacking directed at China -- come amid tensions between Washington and Beijing about online espionage.

Snowden has vowed to resist any attempt by Washington to extradite him from the semi-autonomous Chinese territory of Hong Kong, a former British colony with an independent judiciary and a strong tradition of free speech.

But China is ultimately responsible for its defence and foreign affairs, and the relationship has sparked intense interest in what role Beijing will 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 gave 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 any such request.

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

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

"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.

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 says the vast dragnet is needed to keep Americans safe.

As of Thursday evening Snowden's claims of US hacking targeting China were the top story on the popular Chinese web portals Sina and Tencent, and the second-highest on another leading site, Sohu.

State media quoted analysts as saying disclosure of the surveillance programme could test relations.

The programme "is certain to stain Washington's overseas image and test developing Sino-US ties", the China Daily cited analysts as saying.

"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 Global Times said China had the right to deny any US extradition request if it did not get answers about US spying.

Beijing wielded "veto power on the condition that the extradition affects China's core public interests or policies", it said.

"Snowden's revelations involve China," it said. We "have the right to request the US government to explain.

"If this involves the issue of extraditing Snowden, we could decide whether to cooperate according to different situations."

The paper also criticised the US's apparent hypocrisy in touting freedom and human rights abroad.

"The US especially likes to use these weapons to jab China," it said.

"America's true face and false face, we should both see more clearly."