The search engines of two of the most popular Web portals in China have been blocked in an intensified censorship of the Internet.

Sina and Sohu are the latest victims of Beijing's increasing control of the Internet for having failed to filter certain keywords deemed politically harmful. These include words like Tibet, Taiwan Independence, Falun Gong and the Tiananmen Square crackdown.

China had more than 97 million search engine users in 2004, and this will exceed 100 million this year, according to a recent report by iResearch.

The two portals had been given three days to "rectify their mistakes," the South China Morning Post reported.

The Sina and Sohu search engines have been out of service since noon June 19, with the search pages carrying a message that the sites were undergoing upgrades

Western Internet firms including Microsoft and Yahoo have long been criticized for putting the quest for profits ahead of ethics and agreeing to censor websites and content.

Internet providers operating in the mainland are required to filter content by blocking hundreds of keywords or by denying access to websites considered politically sensitive by the authorities.

The two companies declined to elaborate on the shutdown or provide statistics about their users.

Sohu spokeswoman Zhang Xin insisted that it was normal to have a system upgrade. Her counterpart in Sina, Yan Hongyan , did not offer an explanation. "I don't know yet what has happened and why the search engine is out of service," she said in the Post report.