More than 50 celebrated American and international literary figures have written a joint letter to Chinese President Xi Jinping appealing to his "sense of conscience" to release Liu Xia, the widow of Nobel Peace Prize winner Liu Xiaobo.

Liu Xia, a poet, painter and photographer, has been under house arrest in China since 2010 despite never having been charged with any crime.

She was last seen in public on July 15 when she participated in a memorial service for her husband, two days after he died of cancer. Friends, journalists and diplomats have all been prevented from meeting her. She is reported to be suffering from depression and a heart condition, the letter said.

The 53 writers who appealed for her release, in an open letter organized by PEN America, a group set up to advance literature and defend free expression, include Philip Roth, Paul Auster, Chimamanda Adichie, Louise Erdrich, John Coetzee, Margaret Atwood, Paul Auster, Anne Tyler and Tom Stoppard.

They called for her release in recognition of China's international obligations, including as a signatory to the Universal Declaration of Human Rights, and its own constitution that guarantees freedom of speech and other rights.

"We also appeal to your conscience and your sense of compassion," they wrote to President Xi. "Liu Xia has undergone great suffering for many years, simply for being the wife of a man that China has deemed to be a dissident. She has committed no crime, and she has not been charged with any crime."

Although Chinese officials claim she is free, she is in fact in "de facto incommunicado detention" — cut off from the outside word and barred from making free decisions on whom to speak with and where to travel, the letter said.

"She is in poor health, she is isolated from those who care for her, and she is grieving deeply for the loss of her husband. She should be free to meet freely with family, friends, and members of the international community, free to travel where she wishes, and free to be reunited with the outside world."

PEN America said it planned to call on President Trump to bring the letter with him on his state visit to China next week. The group has also opened it up for signature by members of the public.

Liu Xiaobo died of cancer at the age of in July at the age of 61, seven years after winning the Nobel Peace Prize for his "long and non-violent struggle for human rights in China." In 2009, the Chinese government sentenced him to 11 years in prison for "inciting subversion of state power."

He took part in the 1989 Tiananmen Square pro-democracy protests and was co-author of Charter 08, a manifesto calling for democracy and freedom in China.

The authors said they were adding their voices to those of the U.N. High Commissioner of Human Rights, the Norwegian Nobel Committee, Secretary of State Rex Tillerson and the German Embassy in China calling for Liu Xia's release.

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