Liu Xia, the wife of imprisoned Chinese dissident and Nobel Peace Prize winner Liu Xiaobo, has been admitted to hospital in Beijing with heart problems, a family friend said Thursday.

Liu, who has been under house arrest since 2010 but has not been charged with any crime, was admitted on Tuesday, Mo Shaoping, a human rights lawyer and close friend, told AFP.

The move came less than two weeks after she was admitted to a different Beijing hospital only to be abruptly told to leave, he said.

"They continued looking for a hospital" after the first one turned her away, Mo said.

"Now, the latest news is that Liu Xia has been admitted at a different hospital. But the police won't let her family discuss any of the details."

"Of course, it would be better if she could go abroad to see a doctor," Mo added. "The problem is whether or not they will let her go. They won't let her decide for herself."

Liu Xiaobo was a co-author of Charter 08, a bold petition calling for political reform in the Communist-ruled country, and he was sentenced to 11 years in prison in 2009 on subversion charges. His Nobel award the following year outraged Beijing.

Those close to Liu have expressed increasing concern about her physical and mental state in the three years since she was placed under house arrest. Some worry that her depression has deepened in recent months due to her heavily restricted contact with friends and family.

She has no Internet, is not allowed visitors, may speak by phone only rarely to family and "non-sensitive" acquaintances, and must be accompanied by police whenever she runs errands such as buying groceries, said Hu Jia, a political activist and friend.

"She watches TV. She reads books. Sometimes she spends half an hour or so using a stationary bike on her porch," Hu said. "But that's not to say that her health is good."

Liu Xia last visited her husband at a jail in northeast China last month, and last year she was allowed to leave her house for brief periods to attend the trial on fraud charges of her brother, Liu Hui.

He was sentenced to 11 years in jail in a case that relatives and lawyers have said was politically-motivated retaliation against the family.

"She feels very sorry toward her brother and her brother's wife, because she believes this was a political move," Hu told AFP.

- Ordered to leave -

The most recent public images of Liu Xia were released in January, when a video of her at home was posted online by China's Independent PEN Centre, a group of writers advocating freedom of expression.

In the video Liu, appearing thin and smoking a cigarette, read two self-written poems.

Her continued isolation and long hours locked indoors appear to be taking both a physical and mental toll.

Mo said that after a heart episode in January, Liu was told by authorities she would be able to see a doctor at a police hospital.

Liu declined, saying that she wanted to see a doctor abroad instead, and police eventually allowed her to go to a Beijing hospital of her choosing, Mo said. But the following day hospital officials told her that she had to leave, offering no explanation.

Her friends suggest that either hospital staff were unnerved by the large police presence accompanying Liu or that authorities themselves requested her discharge.

"My personal opinion is that they think if Liu Xia is hospitalised, then journalists and foreign diplomats and close friends of her and Liu Xiaobo might go to see her," Hu said.

"This would put new pressure on the authorities.

"If this is the reason they won't let her seek medical treatment -- a political prisoner of three years -- it's completely inhuman," he added.