A United Nations panel says China's imprisonment of Nobel Peace Prize winner Liu Xiaobo violates international law and he should be freed and compensated for his plight.

In a opinion that was dated May, but released only Monday by a group called Freedom Now, the United Nations Working Group on Arbitrary Detention said Liu was being wrongfully held and should be released immediately. It also called on China to end the house arrest of his wife, Liu Xia, and compensate them both.

Liu has campaigned for decades for freedom of expression in China. His co-authorship of the document Charter '08 led to his most recent detention, which eventually resulted in an 11-year prison sentence. Shortly after, Liu was awarded the 2010 Nobel Peace Prize for his efforts to improve the rights of Chinese citizens. The Chinese government reacted to the prize angrily, even lashing out at Norway, where the Nobel committee is based. As yet, China has not responded to the UN group's opinion.