The three publicly traded companies are ignoring their own stated commitments - which in Google's case includes the corporate motto "Don't be evil" - and are in denial over the human rights implications of their actions, the group said on Thursday.

"All three companies have, in one way or another, facilitated or concluded in the practice of censorship in China," London-based Amnesty said in a report.

"All three companies have demonstrated a disregard for their own internally driven and proclaimed policies. They have made promises ... which they failed to uphold in the face of business opportunities and pressure from the Chinese government.

"This raises doubts about which statements made by these organisations can be trusted and which ones are public relations gestures."

Yahoo! said its presence in China, even with the restrictions, could still help open up the country and said that it, too, was concerned by the issue.

"We believe we can make more of a difference by having even a limited presence and growing our influence, than we can by not operating in a particular country at all," Yahoo! said in a statement to Reuters.

Calls to Google and Microsoft representatives in China were not immediately returned.

Filtering policies

Amnesty said the three companies were in violation of Article 19 of the Universal Declaration of Human Rights, which says everyone should be guaranteed freedom of expression.

The group said it was calling on the three companies and other internet firms to lobby Beijing to release all "cyber dissidents", be open about what filtering policies they operate, and promote human rights in China.

Amnesty said some actions the firms had taken, such as Google's refusal to offer an email service in China due to privacy-invasion worries, were good, but more action was needed.