WASHINGTON (Reuters) - U.S. Secretary of State Rex Tillerson expressed condolences for the death of Chinese dissident and Nobel Peace Prize-winner Liu Xiaobo on Thursday and called on China to release his wife Liu Xia and let her leave the country.

“Today, I join those in China and around the world in mourning the tragic passing of 2010 Nobel Peace Prize Laureate Liu Xiaobo, who died while serving a lengthy prison sentence in China for promoting peaceful democratic reform,” 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.”

Liu Xiaobo, a prominent dissident since the 1989 Tiananmen Square pro-democracy protests, died on Thursday after being denied permission to leave the country for treatment of late-stage liver cancer.

U.S. House Democratic Leader Nancy Pelosi echoed Tillerson’s call for Liu Xia’s freedom and described her husband as “one of the great moral voices of our time.” She said his treatment was “a sobering reminder of China’s shameful disregard for basic freedoms.”

Republican Senator Tom Cotton said Liu Xiaobo has shown “a quiet but formidable courage that will live on in the hearts of all Chinese people who long to be free.”

“To deprive a man of treatment and watch him die a slow, painful death is an outrage against humanity. But the Communist regime was so cruel toward Liu Xiaobo because its leaders knew, in their hearts, the power of his message,” he said.