The United States mourned the death of imprisoned Nobel laureate Liu Xiaobo Thursday and called on China to free the dissident's widow from house arrest and let her leave the country.

"Mr Liu dedicated his life to the betterment of his country and humankind, and to the pursuit of justice and liberty," US Secretary of State Rex Tillerson said in a statement.

"I call on the Chinese government to release Liu Xia from house arrest and allow her to depart China, according to her wishes," he said.