BEIJING (AFP) - The wife of jailed Chinese Nobel Peace Prize laureate Liu Xiaobo suffered a heart attack two months ago and has been diagnosed with depression after years of house arrest, a rights group said Tuesday.

Liu Xia, 53, has not been charged with any crime but has been held under house arrest since 2010, when her husband was awarded the Nobel Peace Prize while serving an 11-year jail sentence for subversion.

Liu Xia suffered a heart attack in January, but was treated at a hospital in Beijing and her physical condition has since improved, London-based rights group Amnesty International said in a statement.

But doctors in Beijing diagnosed her with "psychological stress, anxiety and depression", and warned that her depression was likely to worsen unless her living conditions change, it said.

Friends have said that Ms Liu has endured months of continuous isolation and is rarely allowed to leave her Beijing apartment. Amnesty said she has recently been allowed to speak to friends by phone.

The report comes as China faces new scrutiny over its treatment of dissidents held under detention, following the death of a prominent activist.

Human rights campaigner Cao Shunli died in police detention two weeks ago after she was denied medical treatment, her family and lawyers said.

China denied that she had been mistreated, but her death was condemned by the US and the European Union.

Liu Xiaobo is currently in prison after a court jailed him in 2009 on a charge of "incitement to subversion of state power", based in part on his spearheading of Charter 08, a petition urging greater protection of human rights and democratic reforms in China.

His Nobel award in 2010 incensed China's Communist authorities, which deal harshly with dissidents and other potential threats to their authority.

Liu Xia was allowed to leave her house for brief periods last year to attend the trial on fraud charges of her brother Liu Hui, who was sentenced to 11 years' jail in a case that relatives and lawyers said was politically-motivated retaliation against the family.