Liu had been kept incommunicado by the authorities for almost a month, her lawyer has said.
Lu Siqing, founder of the Hong Kong-based Information Center for Human Rights and Democracy, said he spoke to her by phone at her Beijing home on Saturday.
He said a tearful Liu, 56, explained in a "very weakened voice" that she was under treatment with strong anti- depressants.
"Various friends of Liu Xia confirmed that she was back in her apartment" in the Chinese capital, "and that her home was kept under surveillance by guards and police dressed in civilian clothes," the NGO said in a separate statement.
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Her relatives were unable to contact her in the following weeks.
Liu Xia "is held incommunicado by the Chinese authorities in an unknown location," said the couple's lawyer Jared Genser in a complaint filed with the UN.
A video message posted online in mid-August in which Liu Xia said she needed time to "readjust" was greeted with caution.
"It is certain that she was forced by the authorities to make this video," said Hu Jia, a Chinese dissident and friend of the couple.
But he said she had been unable to obtain the urn that contained her husband's ashes which were scattered at sea.
Liu Xiaobo died on July 13 of liver cancer in a Chinese hospital at the age of 61, after being transferred there from prison.
He was sentenced in 2009 to 11 years in jail for "subversion" for calling for democratic reforms in China -- a fight rewarded in 2010 by the Nobel Peace Prize.
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