Liu Xia was last seen one month ago - the day her husband was buried at sea. No one has heard from her since, and her whereabouts remains unknown.

The Chinese government rejects allegations she is being held against her will. But Human Rights Watch says her prolonged silence raises serious doubts about her freedom.

"It's to keep her from telling the world what's happened to her and what happened to her husband," the rights group's China director, Sophie Richardson, told DW in an interview. "It's precisely to silence her."

Beijing faced an international backlash for its treatment of outspoken government critic and human rights activist Liu Xiaobo, who died in custody last month after battling liver cancer. The 61-year-old was serving an 11-year sentence for "inciting subversion of state power."

Years under house arrest

Rights groups, the United States, the European Union and the United Nations have called on Beijing to free Liu Xia, who has been under house arrest since her husband won the Nobel Peace Prize in 2010, despite having never been charged of a crime.

"This is a problem for the Chinese government, that it has imprisoned for years these people who've done nothing - that's criminal," Richardson said. "This is a major embarrassment for them. And now we know the lengths they'll go to, no matter how barbaric, to keep that story from being told."

Liu Xiaobo, China's best known political prisoner, died last month

Perhaps most concerning, says Richardson, is that Liu could be deprived of proper medical treatment. The 56-year-old poet suffers from depression and has also been treated for a heart condition in the past. "She needs to be allowed her freedom to be treated as she sees fit and any restrictions on that are utterly unacceptable," she said.

'Enforced disappearance'

In early August, Liu's US-based lawyer lodged a formal complaint to the United Nations, accusing the Chinese government of facilitating his client's "enforced disappearance" and holding her "incommunicado in an unknown location."

Chinese authorities insist Liu is a free citizen who is merely too grief-stricken by her husband's death to be in touch with any friends. They've also dismissed inquiries about Liu from the press and other governments as foreign interference in China's domestic affairs.

Germany has been pressing the Chinese authorities to lift any restrictions placed on Liu Xia, and to let her leave the country if she chooses to do so. The widow had reportedly expressed a wish to go to Germany, according to diplomatic sources.

Richardson called on German Chancellor Angela Merkel to keep up the pressure on Chinese officials, and to "really make clear that Germany doesn't just care about trade issues, but that it's also going to push on the Chinese government's abusive treatment of people."