The Chinese government is clamping down on Twitter-like microblogging services in a bid censor political opposition, according to sources quoted by the Reuters news agency.

Twitter itself has been banned for a year. This week, a number of alternative services from sites including NetEase.com and Sohu.com were also shut down.

The companies themselves deny the outages are due to government intervention. "NetEase’s micro-blog is very popular and growing fast, so we had to perform maintenance to upgrade features," a spokesman for NetEase told Reuters.

But the agency quotes unnamed sources saying the closures are the result of a major campaign to tighten Internet controls, although “nobody will publicly announce the reason”. China’s Oriental Morning Post has made similar claims.

Earlier this month, the Associated Press profiled Chinese dissident Yu Jie, who uses Twitter to share his thoughts and experience with sympathisers around the world.

Jie uses a web proxy to access the site. "In China, you have to get around the Great Firewall, so those who use Twitter are usually interested in politics," he says. "For me, it’s a very important way for me to express my political opinions."

Also this month, the Chinese government produced guidelines for using social media in PR. Among its recommendations is to ensure that “content disseminated on the Internet is not inconsistent with objective reality”.