BEIJING - Two more Web sites dedicated to social networking went offline in China yesterday amid tightening controls that have blocked Facebook, Twitter, and other popular sites that offered many Chinese a rare taste of free expression.

China's crackdown on social-networking sites began in March when Web users in the country found they could no longer visit YouTube, shortly after video appeared on the site purporting to show Chinese security officials mistreating Tibetans.

The blockages continued through the 20th anniversary on June 4 of the Tiananmen Square demonstrations and the recent ethnic riots in Xinjiang, with homegrown and overseas microblogging and photo-sharing sites among those targeted.

Although cutting off access to sites can often be traced to a specific trigger - such as the Tiananmen anniversary - experts say the fact that the sites are not coming back online shows that the harsh measures are part of a long-term strategy to pare back the power of the Internet and silence some voices finding expression here.

"I am especially pessimistic about this fall and next spring," said Wen Yunchao, a well-known blogger based in Guangzhou in southern China. "I expect they will be more and more restrictive, because they have yet to come up with a good way to manage the Internet. They are aware that it has this great power and they are afraid of it."

Digu and Zuosa, two Chinese Web sites that offer microblogging services similar to Twitter, were shut down for maintenance yesterday, notices on their home pages said. A Digu spokeswoman who would give only her surname, Zhang, said Digu was offline and would be down at least a week so it could be moved to a new server.

"It's a sensitive period, so we are not in a rush to reopen it," Zhang said.

Some Digu users had recently tried to post politically sensitive material to the site, she said, and the company was having to censor such content. She would not give any specific examples.

Zuosa employees did not respond to an e-mail seeking comment about the site's closure.

Having two sites close the same day indicates pressure from authorities for them to shut down, said Wen, the blogger from Guangzhou. He said the timing was probably related to today's 10-year anniversary of the banning of the Falun Gong spiritual movement.

"Maybe the Chinese government is concerned that the Falun Gong will use the occasion to spread some rumors or organize some kind of event via the Internet," he said.

Also yesterday, the technology channels of the popular Sina and Netease Web portals were shut for about six hours, apparently because they had posted news about a corruption probe without clearance from state censors.

China has the world's largest population of Internet users, more than 298 million, and the world's most extensive system of Web monitoring and censorship. Despite those controls, the Internet's role as a platform for sharing unofficial news and opinions has expanded rapidly.