The disclosure of a huge US electronic surveillance programme will test US-China relations already strained by Washington's accusations of cyber-spying by Beijing, state media said on Thursday.

Chinese media had remained relatively quiet during a public holiday about bombshell revelations by a former US government subcontractor of massive phone and Internet spying.

Adding to the diplomatic complications, the 29-year-old source of the information, Edward Snowden, has flown to the semi-autonomous Chinese city of Hong Kong and vowed to resist extradition.

Snowden also told Hong Kong's South China Morning Post on Wednesday that the US National Security Agency (NSA) has hacked computers in China and Hong Kong since 2009.

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."

An analyst cited by the paper noted the irony that the US's surveillance programme was exposed just as Washington has intensified its public accusations of Chinese state-backed cyberattacks -- an allegation which Beijing vehemently denies.

The two sides' presidents Barack Obama and Xi Jinping discussed the issue at a two-day summit in California last week.

"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.

Leaks and reports have 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 huge debate about privacy and security.

As of Thursday afternoon Snowden's claims of US hacking targeting China were topping the headlines on the leading Chinese web portals Sina, Sohu and Tencent.