China's state news agency called on Internet sites Tuesday to stop the "cancer" of online rumours, in the latest sign of official unease over the rising popularity of social networking sites.

The Xinhua news agency's call for an end to online "rumour-mongering" came days after a similar warning from a senior Communist Party official and reflects the government's growing disquiet at the rapid rise of China's micro-blogs.

"The Internet is an important carrier of social information, civilization and progress. Rumours will harm the network and are a dangerous cancer," Xinhua said in a commentary published only in Chinese.

"The reform of Internet technology has facilitated the rapid flood of information and communication but also has brought... serious cyber rumours, that harm the Internet's development.

"To nurture a healthy Internet, we must eradicate the soil in which rumours grow."

China, which has the world's largest online population with 485 million users, constantly strives to exert its control over the Internet, blocking content it deems politically sensitive as part of a vast censorship system.

But the rise of China's weibos -- microblogs similar to Twitter, which is banned by the communist authorities -- has exposed the difficulty of controlling access to information.

After a deadly train crash in July, Sina's Weibo users sent millions of messages criticising the official response to the disaster, which killed 40 people and forced the government to halt the expansion of high-speed rail.

The scale of the response appeared to take authorities by surprise. Shortly after the accident, the People's Daily, the mouthpiece of China's Communist Party, urged officials to use the weibos more to communicate with the public.

While official media must clear its news before going public, censors have weaker means to control what is said and shown on weibos, where users can post commentary, photographs and video.

Though censors, many employed by the companies themselves, erase offending messages as rapidly as they can, some stay on the web for hours or days before they are caught.

Last week, senior Communist Party official Liu Qi reportedly visited the offices of China's top Internet companies to urge them to stop the spread of "false and harmful information."