The US government has formally asked China to turn over details of its policies for censoring websites.

US Trade Representative Ron Kirk said on Tuesday that the request is being pursued under World Trade Organisation rules governing how member countries deal with trade issues. Kirk said the concerns center on the competitiveness of foreign websites in China.

China heavily censors its Internet, and Kirk said the US wants to understand China's rules governing website blocking so that foreign websites can adopt policies to avoid being blocked.

The request also seeks information on the mechanics of Internet censorship in China, and whether it's implemented directly by the government or indirectly by Internet providers.

Some 500 million people in China use the Internet and are hotly pursued by marketers. China's censorship rules are opaque, and it's not just political sites that are blocked.

China's policies led to a rare public fight with a major US company last year when Google Inc announced it was going to stop censoring its Internet search results on its Chinese website as the result of a hacking attack believed to have originated in China.

The US is increasing pressure on China over its trade policies, including its currency practices, but there is broad disagreement about the appropriate strategies for ensuring fair competition.