Websites run by Chinese human rights activists were hit by cyber-attacks over the weekend, highlighting increasing sophistication of efforts to censor dissident voices in China.

Hackers set launched a distributed denial of service (DDoS) attack that lasted 16 hours starting Saturday afternoon, according to the advocacy group Chinese Human Rights Defenders, which was also attacked.

The Independent Chinese Pen, New Century News, Canyu and Civil Rights and Livelihood Watch were among the other websites that were disabled from the coordinated attack.

The group said it could not confirm the origin of the attacks but suspected official censorship on part of the Chinese government.

I don't think ordinary Chinese hackers have this kind of capability, said an IT expert associated with CHRD. The Chinese government is the most likely suspect for these attacks.

A DDoS attack coordinates legions of computers to visit a targeted website at once, overwhelming its server and leaving the site inaccessible to normal visitors.

Hackers also placed malware on two of the Web sites before the attack, but that is now being removed, the statement said.

The attack comes on the eve of calls from the US for world governments to ensure the freedom of its citizens to access content and websites freely.

The freedom to connect is like the freedom of assembly in cyberspace, US Secretary of State Hillary Clinton said last week.

Clinton's comments come in the wake of a threat from Google to pull its operations out of China after it too was a target of cyber-attacks originating in country.

Foreign journalists in China last week also said the Google email accounts of at least two reporters there had been recently hijacked.

Clinton call for international condemnation of countries and individuals that engage in such hacking was roundly dismissed by China.

China's foreign ministry spokesman Ma Zhaoxu, sternly rebuked Clinton's speech, calling for the US to respect the facts and stop using the issue of so-called Internet freedom to unreasonably criticize China.