WASHINGTON (AFP) - The leading U.S.-based Internet companies are showing little interest in attending a Congressional briefing on worries that the firms are bending to the wishes of China's censors.

Microsoft and Cisco Systems have refused to attend the event, while Google and Yahoo are non-committal, officials said.

The firms were asked to attend the February 1 briefing by the Congressional Human Rights Caucus following uproar caused by search giant Google's decision last week to censor websites and content banned by China's propaganda chiefs.

"We have heard from Microsoft that no representative from the company will attend the briefing. So, with Cisco Systems, this makes two companies that have confirmed they're opting out," Lynne Weil, spokeswoman for caucus co-chairman Democratic Representative Tom Lantos, told AFP.

As the briefing date gets closer, "others are still unfortunately keeping us in suspense," she said. "It is mystifying why these companies would not want to take part after all this is an opportunity to clear their names," Weil said.

Google agreed to censor websites and content in its search service launched Wednesday in China.

The move followed similar actions by rivals Microsoft and Yahoo in cooperating with Chinese censorship.

Cisco's technology-savvy machinery allegedly censors Internet messages and helps Beijing track down Chinese cyber dissidents.

Americans believed Internet growth will bring about greater freedom of expression and political openness in China but instead of promoting these values, some U.S. firms have been charged with aiding -- or at least complying -- with Chinese Internet censorship, said a statement by the Congressional Human Rights Caucus.