Chinese social networking websites that provide Twitter-like services have suddenly reverted to testing mode and access has been spotty amid reports of a government clampdown.

Although Twitter has been banned for more than a year in China, Chinese Internet companies have been quick to fill the void, providing microblogging services that allow users to post frequent updates and follow other posters.

On Wednesday, NetEase.com Inc's microblog (http://t.163.com) was inaccessible. A notice said the site had been down since 7 p.m. on Tuesday and was under maintenance.

Sohu.com Inc's microblog (http://t.sohu.com) was also shut down for more than a day earlier in the week and all Chinese twitters now display the notice in testing mode.

Company sources told Reuters that the developments were the result of tightened government controls over the new services.

Nobody will publicly announce the reason, but it is as obvious as a fly on a bald head, one source said, declining to be named because of the sensitivity of the matter.

The Shanghai-based Oriental Morning Post cited unnamed industry sources as saying that the websites were under pressure from Chinese censors.

News content on Chinese Internet websites is under intense government censorship, and online news editors with major Internet portals often receive dictats from the government on what can and cannot be published. But the new microblogs of Internet portals, with less government censorship, have proved to be freer for carrying news and comments.

Nevertheless, in July 2009 Fanfou.com, a budding replica of Twitter, was shut down by the government amid a major campaign to tighten Internet controls.

Beijing has been trying to tighten controls on the country's booming Internet industry, the world's largest by users, since the second half of last year, introducing new regulations concerning online gaming, online mapping and e-commerce.

Sina Corp, China's largest Internet portal, launched a microblog in August last year. Yet, earlier this week, the company put an in testing mode notice on the website.

We are constantly upgrading the site. Even though we launched in August last year it is still in testing mode, said Sina spokesman Liu Qi.

Sina and NetEase both denied government intervention and said the notices and sporadic site access were due to upgrading of features.

NetEase's micro-blog is very popular and growing fast, so we had to perform maintenance to upgrade features, said NetEase spokesman Liu Youcai.

Sohu could not be reached for comment.

(Editing by Chris Lewis and Alex Richardson)

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