There are also claims Cisco has assisted the government in tracking down cyber-dissidents.

All four companies refused to participate in the briefing but Microsoft and Yahoo did release a joint statement calling on the US government to help solve censorship problems in China.

"We urge the United States government to take a leadership role in this regard and have initiated a dialogue with relevant US officials to encourage such government-to-government engagement," the statement said.

Anti-censorship groups, Reporters Without Borders, Amnesty International and Human Rights Watch want Congress to introduce a uniform code of conduct to protect freedom of speech.