Prominent Chinese human rights lawyer Gao Zhisheng has been sent back to prison after a court ruled he had violated the terms of his probation, the official Xinhua news agency said Friday. The move was criticised by the United States who urged China to immediately release Gao and to clarify his whereabouts, which have been unknown for months. 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. The Beijing court sent him back to jail for three years after ruling he had "seriously violated probation rules a number of times", the report said Friday -- less than a week before his probation was due to expire. AFP calls to the court went unanswered. 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. China's foreign ministry has repeatedly denied any knowledge of Gao's whereabouts despite persistent questioning by journalists. US State Department spokeswoman Victoria Nuland told reporters the United States was "deeply disappointed" by the announcement that he had been sent back to prison 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." Human rights activists said the court's decision showed Beijing was determined to prevent Gao speaking publicly about his time in detention -- if in fact he was still alive. Nicholas Bequelin of Human Rights Watch said: "There's nothing there (in the Xinhua report) that tells you he is alive and well." "At the very last minute... they decided they couldn't let the world see him or hear him and decided to take him away for another three years." Earlier this year a United Nations human rights agency called on Beijing to free Gao, who has previously been touted as a possible Noble Peace Prize winner. The European Union has also raised Gao's case with  Chinese authorities and this latest development is likely to spark a fresh protest. Amnesty International called the Beijing court ruling against Gao "truly shocking" and demanded that he be freed immediately, saying that he and his family have "suffered enough". "The authorities' belated attempt to cast a veneer of legality over their treatment of Gao Zhisheng is truly shameful," Catherine Baber, the London-based rights group's Asia-Pacific deputy director, said in a statement. "We urge the international community to continue to press the Chinese government for Gao's release. The international community must not let up in their condemnation of this travesty of justice," she said. Gao's troubles began more than five years ago, when he renounced his Communist Party membership and openly called for an end to a crackdown on the banned Falungong spiritual group. In December 2006, he was convicted of subversion and given a suspended sentence of three years in prison, immediately placed under house arrest and put on probation for five years. In April 2010 Gao gave a dramatic account of a brutal two-day beating by police during an interview with the Associated Press, which was only published in January. The dissident described how he was stripped naked and violently pistol-whipped for two days, during which his life "hung by a thread". The agency, which said Gao had asked that the story not be published unless he went missing again or found safe haven abroad, said it decided to release details from the interview due to Gao's prolonged disappearance. "That degree of cruelty, there's no way to recount it," Gao said, adding the beating he suffered at the hands of three police officers in September 2009 was the worst he had endured in 14 months of detention that began early that year. "For 48 hours, my life hung by a thread." Gao's wife, Geng He, fled to the United States in early 2009 with their two children. The plight of human rights activists in China has come under the spotlight since the Nobel Peace Prize in 2010 was awarded to jailed dissident Liu Xiaobo, with the West pressing for the release of all political prisoners.