Xinhua news agency said the sites were the first casualties in the campaign announced on Monday and which has already seen major Chinese and foreign Internet sites apologise and take steps to clean up.

'There are still some websites that are not doing enough. Relevant government departments are now thoroughly checking and further raising the strength of the clean-up,' Xinhua said.

The report gave few details on the sites that were shut.

China's Ministry of Public Security and six other government agencies launched the drive against sites that post or link to content that 'harms public morality' and corrupts the nation's youth, Xinhua said.

The have included Google, MSN and Baidu, the most popular Chinese search engine.

Companies that ignore government warnings to remove obscene content or links to such material have been threatened with closure.

Google, Baidu and other Internet portals have since issued apologies and moved against online porn.

Google said on Wednesday it had deleted all links to vulgar material from its search indexes and go all-out to prevent such material re-appearing.

'Google is willing to be a law-abiding citizen in China,' it said in a statement.

China has launched Internet crackdowns on pornography, con artists and political activists in the past, but officials have warned the latest campaign would include tougher measures, without giving specifics.

China has the world's largest online population at more that 250 million, according to official figures, and it is growing rapidly as computer use rises along with income levels.

However, some of the Internet companies named have said it would be difficult for them to immediately spot and eliminate all obscene content due to the nature of Internet search engines.

China's communist rulers generally exercise strict control over the Internet, blocking sites linked to many politically sensitive subjects. -- AFP