Amnesty International has challenged Google, Yahoo and Microsoft to reveal the words censored by their content filters in China.

The human rights organisation blasted the web giants for "facilitating or colluding in the practice of censorship in China", and overlooking their human rights obligations in order to tap into China's lucrative online market.

In a new report, Amnesty also urges the companies to make public their censorship agreements with the Chinese government, and to lobby Beijing for better human rights protection.

Yahoo came in for the harshest criticism. "Yahoo's actions have, in particular, assisted the suppression of dissent with severe consequences for those affected," said the Amnesty report.

The group has criticised the three companies before. Yahoo came under fire after allegedly handing over user information that helped Chinese authorities identify and jail dissidents.

Microsoft was criticised for shutting down an MSN Spaces blog following a request from the Chinese government. 

The Amnesty report welcomes some steps taken by the web firms, however, applauding Google for admitting that its business practices in China are at odds with its corporate mantra of 'Don't be evil'.

Amnesty also concedes that Google Ch ina does at least let Chinese users know when information has been removed from its search results, although it does not, of course, say what that information might be.

The search company has claimed that this provides some transparency and "is a step in the right direction". 

However, Amnesty said that that these steps do not go far enough, and has asked the firms to reveal exactly what they censor.

Google responded to the report by saying that Google China benefits Chinese internet users and expands their access to information. In other words, it's better than nothing.

This follows Google founder Sergey Brin's insistence last month that the company would stay in China despite rumours to the contrary.

Representatives for Yahoo and Microsoft in Asia and Europe have declined to comment. In the past, they have defended their business practices by pointing out that they must follow Chinese laws.