Beijing (CNN) -- Don't post rumors -- or else.
That was Sina.com's message to millions of micro-bloggers recently, when China's leading content portal warned it will penalize users if they find them spreading false rumors on their Weibo account.
Weibo is Sina's Twitter-like social media service, which boasts of 200 million registered users.
Speaking at the China Digital Media Summit recently, Sina.com's CEO Charles Chao pledged to curb irresponsible rumors. "Weibo is a microcosm of a big society and a society needs to be properly managed by regulations," he said.
Chao said Sina has been working to set up a "credibility system," which would rate Weibo posts. This, Chao said, will "spread the real valuable information and punish those who make up rumors." The punishment includes imposing a temporary freeze on Weibo accounts, from one week to one month.
Observers of China's new media industry say this is another attempt by Beijing to tighten the control of the internet.
Earlier this year, Chinese bloggers battled through targeted internet censorship in the wake of dissident artist Ai Weiwei's release after nearly three months in police custody.
On Weibo words with the slightest linkage to Ai were banned, including "release," "AWW" and "the fat guy." The phrase "love the future," which looks and sounds like his name in Mandarin, was also blocked.
"There has been government concern about the way the internet spreads information for years," says Beijing-based media observer Jeremy Goldkorn. "Sina, like all internet companies, has no choice but to work with the government, which is something they have always been good at doing."
In August, Sina sent out "notices" to all users citing reports circulating in cyberspace that were untrue. One claimed that the Red Cross Society has been selling blood to Chinese hospitals at 200 yuan per pack even though it had been donated for free.
Weibo also set up an account intended to refute malicious rumors circulating among netizens. In a public statement issued in early September, Sina.com said Weibo Piyao would "ensure the authenticity" of information across the site.
"After the Wenzhou train accident, which saw a frenzy of angry posts on Weibo, the government wants internet companies to make sure that their websites do not pose any threat to social stability," said Goldkorn.
The high-speed train collision killed 39 people and sparked a massive outpouring of anger directed at officials for their handling of the crash. Much of that outrage played out on Weibo.
Weibo has long played host to fierce online debates about corruption and social injustice in China -- a delicate balancing act between meeting the expectations of a Web audience accustomed to speaking their minds and not offending the Chinese government to the point of a shut-down.
"Sina and other companies will have to regulate themselves and decide, in real time, what kinds of information to police," said Goldkorn.
But he said it is unclear what criteria would be used to identify the rumors to censor.
In a recent interview with CNN, the Sina.com founder declined to offer specific numbers about how many Sina employees were managing and censoring Weibo content, but he conceded that "there are people working in terms of looking at the content itself and the message itself.
"There are a lot of rumors on the micro-blog itself, a lot of fraud on the micro-blog. There are a lot of things we need to take care of."
As if to reinforce that point, Beijing Communist Party chief Liu Qi recently visited Sina and Youku.com, China's leading video sharing website. According to the Beijing Daily, he urged them to "firmly end fake and harmful information" and to use new technology to "solve problems of online video management."
He added: "Internet companies should work to get rid of damaging or untruthful information and help to create a healthy positive online environment."