BEIJING (REUTERS) - The wife of jailed Nobel Peace Prize Laureate Liu Xiaobo is unwell under house arrest and possibly suffering from severe depression, but refuses to seek medical help as she is afraid of further punishment, her friends said on Monday.

The accounts from Liu Xia's friends shed a rare light into her condition since being held at home after Liu Xiaobo won the Nobel Prize in 2010.

They also come two days ahead of a visit to China by US Vice President Joe Biden, during which human rights will likely be raised amid a broader crackdown on dissent and freedom of speech and assembly.

Liu Xia wrote to prominent human rights lawyer Mo Shaoping in August that she was "close to going crazy, close to mental collapse" during the time of the trial of her brother, Liu Hui, on fraud charges, Mr Mo said.

"Her family is very worried and they've been giving her anti-depressants," Mr Mo said. "It's been long-term and sustained - when you are cut off from the world for so long, how do you think your mental state would be like?"

Mr Mo said Liu Xia fears officials may force her to accept government-appointed doctors, citing how the government has previously used mental institutions to lock up dissidents.

Mrs Liu is rarely allowed out of her home, except for occasional visits to her husband and family, and is almost never permitted visitors. She has not been convicted of any crime.

Dr Hao Jian, who teaches at the Beijing Film Academy and is a friend of Mrs Liu, said a friend saw her two days ago, when Mrs Liu stood wordlessly weeping beside her apartment window.

"Based on our observations, it is getting more and more serious," Dr Hao said.

"Before she could speak but now she can't say anything." Dr Hao, who saw Liu Xia in March, said Liu Xia was "extremely terrified and does not dare to consult doctors".

Dr Xu Youyu, a professor with the Chinese Academy of Social Sciences, a government think-tank, said he felt Liu Xia's"mental state was on the verge of collapse" the last time he saw her last December.

"She kept on repeating that the stress she was suffering is unthinkable," he said.

Liu Xiaobo, a veteran dissident involved in the 1989 Tiananmen Square pro-democracy protests crushed by the Chinese army, was jailed in 2009 for 11 years on subversion charges for organising a petition urging the overthrow of one-party rule.

Liu Xia last month filed an extraordinary appeal for her husband's retrial, in a move that could renew the focus on China's human rights record.