Offshore operators of Chinese-language porn websites are making huge profits, Chinese government officials said earlier this week.

Compared to local sites, the foreign sites tend to have a wider variety of content, including videos, officials said.

"The boom in pornographic content on the internet has contaminated cyberspace and perverted China's young minds," said Zhang Xinfeng, vice minister of China's Ministry of Public Security, announcing a crackdown on porn earlier this year.

However, Li Jiaming, director of the China Internet Illegal Information Reporting Center (CIIRC) warned that it will not be easy to shut down the overseas sites. 

Chinese users pay to use the porn sites, often using Chinese online prepayment services similar to PayPal.

The CIIRC has persuaded more than 4,000 domestic websites to ban links to the overseas porn sites, Li told local media.

The move is part of a more general crackdown on online porn and other websites which contain content that is illegal under Chinese law.

Of more than 450,000 complaints about potentially illegal websites received by the CIIRC, half related to pornography.

China has strict laws preventing the publication of porn, but enforcement has proved difficult owing to high demand and the huge sums that can be earned by publishers.

It is estimated that more than 60 per cent of the pornographic sites viewed in China are hosted on overseas servers, and therefore out of the government's reach, the CIIRC reported.

The site operators are often located in China, Li believes, but are not easy to track down.

The government is attempting to block their sources of income by asking China's online payment system operators to review sales account applications more carefully.

China uses a nationwide firewall to block access to a large number of websites that contain content which the government deems unsuitable. Many of these are political in nature, but also include sites dealing with religion and pornography.

It is not clear why this approach is failing to prove effective against the thousands of sites targeted in the latest crackdown.