US calls for release of Nobel laureate Liu Xiaobo

Five years after his detention, the US is calling for the release of Nobel Peace Prize winner Liu Xiaobo and an end to his wife's undeclared house arrest.

Washington remains deeply concerned about the couple's treatment, along with that of other jailed government critics, US Secretary of State John Kerry said.

"We strongly urge Chinese authorities to release Liu Xiaobo, to end [wife] Liu Xia's house arrest, and to guarantee to Liu Xiaobo and his family members all internationally recognised human rights protections and freedoms," Kerry said in a statement issued on Monday in Washington.

"As the United States builds a constructive relationship with China, US leaders will continue to raise concerns related to respect for the rule of law, human rights, religious freedom, and democratic principles with their Chinese counterparts."

Detained on December 8, 2008, Liu Xiaobo, was convicted of subversion in 2009 and sentenced to 11 years in prison for penning an appeal for democracy. He was awarded the peace prize in 2010.

Deeply embarrassed, China's government angrily denounced the award and suspended much of its economic and diplomatic contact with Norway, where the prize is awarded.

Kerry's statement underscored a desire to be seen as active on Chinese human rights in the face of critics who say the issue is being downgraded because of Washington's desire for strong trade ties and Chinese support on knotty diplomatic issues such as the conflict in Syria and Iran's nuclear programmes.

Beijing has rejected Washington's appeals as interference in its domestic affairs and refused to offer any legal basis for the house arrest of Liu Xia, who friends say has grown despondent at being virtually confined to her apartment for the past three years.