Friends of the late Chinese democracy advocate and Nobel laureate Liu Xiaobo voiced concern about his widow's health on Thursday after she sent a letter showing signs of deep depression.

The poet Liu Xia, 56, has been under police watch without charges since her husband was awarded the Nobel Peace Prize in 2010, a recognition that deeply angered the Communist regime.

In a letter written in the form of a poem to the 2009 Nobel Literature Prize Laureate Herta Mueller, Liu said she was "going mad".

"Too solitary / I have not the right to speech / To speak loudly / I live like a plant / I lie like a corpse," the poem read.

Exiled Chinese dissident and author Liao Yiwu posted a photo of the letter on his Facebook account on December 9.

The Chinese handwriting appeared to match previously published letters from Liu, who has been under de facto house arrest in her Beijing home for the past seven years.

"I shared her words in the hope of urging Western governments to talk with the Chinese government on this issue and let her go as soon as possible," Liao told AFP by phone from Berlin.

Liao said the widow had sent the poem "recently", but declined to say how she was able to get it out.

"She is taking a lot of medicine to control her depression. If she doesn't take medicine, her heart will jump like crazy. She fainted once," he added.

Another friend, who declined to give his name because he lives in Beijing, said he has not been able to reach Liu since August.

"She must be under tight police control," he said.

The United States and European Union have called on President Xi Jinping's government to free the widow and let her go abroad.

"Foreign governments should press for Liu Xia's release publicly and at the highest level to let the Chinese government know that she is not forgotten," Human Rights Watch researcher Maya Wang told AFP.

Her husband was a veteran of the 1989 Tiananmen Square protests and was detained in 2008 after co-writing Charter 08, a petition calling for democratic reforms.

Following Liu Xiaobo's terminal cancer diagnosis, the democracy advocate requested to receive treatment abroad -- a wish that friends believe was in reality for his wife's sake.

But the authorities refused to let him go and he died in July this year.

Source: AFP