State Department spokeswoman Victoria Nuland told reporters the United States is "deeply disappointed" by the announcement to send him back to jail just as his five-year suspended sentence was set to expire next week.

"We are especially concerned about Gao's welfare and whereabouts, including reports that his family has been unable to communicate with him," Nuland said.

"And we reiterate our calls for the Chinese government to immediately release Gao from custody and clarify his whereabouts."

A Beijing court sent Gao back to prison for three years after ruling he had violated the terms of his probation, the official Xinhua news agency said Friday.

Gao -- who defended some of China's most vulnerable people including Christians and coal miners -- was arrested in February 2009 and has been held incommunicado by the authorities.

He briefly reappeared in March last year when he was apparently released by police, speaking with a few friends and colleagues, many of whom reported that he continued to be tailed by authorities and was in ill health.

A month later, he disappeared again and has not been heard from since.

Recalling remarks made by Gary Locke, the US ambassador to Beijing, on International Human Rights Day last week, Nuland said: "The forced disappearance of Gao is a serious human rights concern.

"It demonstrates that China is not living up to its commitments under the universal declaration of human rights," she said.

"We again express our deep concern over the continued use of extra-legal measures against Gao and other human rights activists and urge China to uphold its internationally-recognized obligations," she said.

The city government now requires users of weibos -- the Chinese version of Twitter -- to give their real names to website administrators, its official news portal said.

The new rules will apply to weibo operators based in Beijing, which include Sina -- owner of China's most popular microblogging service which has more than 200 million users -- as well as users living in the Chinese capital.

Weibo users reacted angrily to the new rules, saying this was an attempt to muzzle online criticism and debate.

"It is good to be responsible for one's own comments but it shouldn't be used as a tool to suppress people's rights," a blogger called Yuyue Yuanfei Ilu said in a posting.

Another web user called V Luoluo said: "The rules are always set against people. Do you dare to tell the truth after the real-name system is implemented? Do you dare to offend someone?"

Weibo operators "must establish and improve a system of content censorship", according to the new rules, while users will have a legal duty to use their true identity to register.

With more than half a billion Chinese now online, authorities in Beijing are concerned about the power of the Internet to influence public opinion in a country that maintains tight controls on its traditional media outlets.

Ordinary Chinese are increasingly using weibos to vent their anger and frustration over official corruption, scandals and disasters.

A weibo user is believed to have broken the news of a deadly high-speed rail crash in China in July that provoked widespread condemnation of the government -- much of it online.

This week, despite attempts to censor the web and a virtual blackout in China's state-run media, weibos have buzzed with news of a protest involving thousands of villagers in the southern province of Guangdong.

Residents in Wukan, which has been under police blockade, have posted information and photos online of their daily rallies to demand justice over land seizures and a local leader's death.

"It's about enhancing control on the weibos. In all likelihood, this registration could make people more cautious," David Bandurski of the China Media Project at the University of Hong Kong, told AFP.

Leading Internet and technology firms have already been pressured to tighten their grip on the web as Chinese leaders try to keep a lid on social unrest in the lead up to a once-in-a-decade leadership transition that begins next year.

Last month the heads of 40 companies, including e-commerce giant Alibaba, search engine Baidu and Sina, vowed to stop the "spread of harmful information" on the web after attending a three-day government workshop.

The seminar was held after propaganda chief Li Changchun, fifth in the Communist Party hierarchy, met the heads of China's main search engine Baidu in September.

That same month, the head of Sina said the web giant had set up "rumour-curbing teams", apparently in response to government pressure. Authorities already have the means to track down web users they believe have broken the law.

Earlier this month two men were detained in the central province of Hunan for "spreading a rumour" that thousands of police officers were deployed to guard a wedding convoy.

State media said the two men posted a video clip online showing scores of police officers and a wedding convoy on a street, which later went viral.