The search engines of the two World Wide Web portals, Sina.com and Sohu.com, were shut down Monday, but appeared to be running again on Wednesday, following a wave of protests by free-speech advocates.
China routinely blocks Internet sites that refer to banned topics such as democracy, the Falun Gong spiritual group, Taiwan independence, and the Dalai Lama. VOA's website is permanently blocked in China.
Jiao Guobiao is a former journalism professor at Peking University who was suspended last year for criticizing China's propaganda authorities. He tells VOA the blocking of Sina and Sohu reflects the government's concern that the Internet is giving Chinese people more freedom to express discontent.
Jiao says the tighter restrictions suggest the government is facing a tough challenge in maintaining control of what appears on line.
"They think they can still use the methods of managing newspapers and magazines, managing the old media, to manage the Internet," Jiao says. "So, in the last couple years - because of controls on the Internet - there have been more and more prohibitions, more and more detailed (controls). If it can be controlled, they have looked for a way to control it."
The Chinese foreign ministry on Tuesday declined to comment on reports that the two portals had been shut down. A ministry spokeswoman defended China's practice of managing the Internet, saying only that it is a legal and rational policy.
The Chinese government employs tens of thousands of people to monitor Web activity, and a number of Web users have been jailed because of e-mails or postings that authorities found objectionable.
The Communist censors have received help from outside. The U.S. Internet firm Yahoo has come under international criticism for aiding the crackdown by handing over information that led to the jailing of Internet users. The U.S. search engine, Google, has voluntarily blocked access to topics the Chinese authorities deem dangerous.
The government's control over cyberspace is being tested this week by New York Times columnist Nicholas Kristof. He has created new blogs discussing sensitive topics such as the 1989 Tiananmen Square massacre, the imprisonment of a New York Times researcher in Beijing, and he has posted a message challenging President Hu Jintao to disclose details of his personal wealth.
At least one of the blogs appeared to be blocked in China on Wednesday. However, Kristof writes that - to his astonishment - most of the material has slipped past the censors.
With technology improving and the number of Chinese Internet users surpassing 120 million last year, analysts say Kristoff's experiment is a small indication of how difficult it is becoming for China's government to keep control of the Internet.