The Chinese government has named 19 web sites, including Google.com, as failing to remove "vulgar" material such as pornography which it says is harming its citizens.
The government list, which also includes Baidu, China's largest search engine, was published as part of a new year crackdown on sites which the Chinese government deems offensive. It has already announced bans on adult video and audio products.
"Some web sites have exploited loopholes in laws and regulations," Cai Mingzhao, a deputy chief of the State Council Information Office, said in a report on an official news web site.
"They have used all kinds of ways to distribute content that is low-class, crude and even vulgar."
Chinese state media is reporting that officials have conducted searches on Baidu and Google that returned too many links to pornography, and warned that the state would take action if the companies did not filter their results.
China already has one of the most heavily censored internet services in the world, and routinely filters sites dealing with politics or certain aspects of Chinese history such as the Tiananmen Square massacre.
Representatives from Google and Baidu are reportedly examining their output to assess the scale of the issue, but have released no formal statements.
China has also announced that it is to strengthen its laws on computer hacking. A draft amendment under review by the Standing Committee of the National People's Congress would impose steep fines and prison sentences of three to seven years, depending on the severity of the offence.
Last week China announced its largest ever piracy bust, in which 11 people were jailed for copying Microsoft software.
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