BEIJING - China is to close unregistered China-based domestic Web sites and blogs, a media watchdog said, as the government tightens its grip on the Internet.

Popular domestic Web portals are already pressured not to publish sensitive news and voluntarily patrol chat rooms and other areas of their sites for "politically incorrect" statements and delete them. Beijing announced in March that every China-based Web site has to register and provide complete information on its organizers by June 30 or face being declared illegal, the Paris-based Reporters Without Borders group said in a statement seen on Tuesday.

"The plan is all the more worrying as the government has also revealed that it has a new system for monitoring sites in real time and spotting those that fail to comply," Reporters Without Borders said.

"This decision will enable those in power to control online news and information much more effectively."

Around three-quarters of domestic Web sites had complied with the registration orders, Reporters Without Borders quoted official Chinese figures as saying.

A report released by the OpenNet Initiative in April called China the world's leading censor of the Internet and said the government employed thousands of officials and private citizens to monitor and control online content.

But for all of Beijing's efforts to rein in the medium, pockets of free speech have appeared in Internet chat rooms and blogs.

"The authorities also hope to push the most outspoken online sites to migrate abroad, where they will become inaccessible to those inside China because of the Chinese filtering systems," Reporters Without Borders said.

Beijing regularly blocks access to some foreign Web pages, including sites run by Chinese dissidents living in exile abroad.

China is the world's second-largest Internet market, with about 100 million users and the number is growing.

It is also the world's largest jailer of cyber dissidents, having detained more than 60 people for expressing their views online, according to a Reporters Without Borders report from last June.