Dissident Chen Guangcheng on Wednesday urged China to release his nephew after a UN group agreed with charges that his imprisonment was arbitrary.

The UN Working Group on Arbitrary Detention, which investigates complaints of wrongful imprisonment, studied Chen's assertions that authorities in Shandong province retaliated against his nephew Chen Kegui after the blind-since-childhood activist escaped house arrest for the safety of the US embassy in April 2012.

The Working Group urged China "to take the necessary steps to remedy the situation, which include the immediate release of Chen Kegui and to grant him compensation for the harm he has suffered during the period of his arbitrary detention."

The opinion was reached in April but released Wednesday by Freedom Now, the legal advocacy group that filed the case. The UN body in Geneva confirmed the finding.

Chen, who was allowed to leave for the United States after appeals from then secretary of state Hillary Clinton on a visit to Beijing, said that mistreatment of his family violated the promises China made during the diplomatic showdown.

"It is time for the Chinese government to correct this mistake by releasing Chen Kegui unconditionally and immediately. It is now obvious that the Chinese government has violated international law," Chen told reporters at the US Capitol.

Nancy Pelosi, the leader of President Barack Obama's Democratic Party in the House of Representatives, said that the measures against Chen Kegui "are not the actions of a confident government."

"To abuse and intimidate the families of a human rights activist, and to do so with impunity, is wrong," she said.

Chen Kegui was sentenced to more than three years in prison in late 2012 on charges of assaulting officials.

The family countered that Chen acted in self-defense, inflicting only light injuries with a kitchen knife, after authorities stormed his home and severely beat him.

China, which usually bristles at international criticism, submitted its position to the Working Group. It stated that there was "no factual basis to the allegation that Chen Kegui acted out of self-defense."

Jared Genser, the founder of Freedom Now, highlighted China's submission, saying: "The Chinese government decided to fight this case at the United Nations... They made their arguments and, quite simply, they lost."

Chen Guangcheng had infuriated authorities in Shandong province by exposing forced abortions and sterilizations under China's one-child-only policy.

He was freed from prison in 2010 after serving a term of more than four years but was put under house arrest, where he reported beatings in a bid to keep him quiet.