Image copyright Reuters Image caption Chinese authorities insist that Liu Xia is a free citizen

The widow of China's Nobel Peace Prize winner Liu Xiaobo has said she is ready to die in protest at being held under house arrest by Chinese authorities.

Liu Xia, 57, has been under house arrest since 2010, after her husband was awarded the Nobel prize. She has never been charged with any crime.

Liu Xiaobo was one of China's foremost pro-democracy campaigners and a fierce critic of the state.

He died last year while serving an 11-year jail sentence for "subversion".

There has been growing concern for Ms Liu since her husband's death.

The poet is said to be suffering from depression after spending years under heavy surveillance, and her friends and lawyer say they believe she is being held "incommunicado". Journalists have been blocked from visiting her.

Advocacy groups have for years called on Beijing to free Liu Xia but the Chinese authorities insists that she is a free citizen, and that the grief induced by her husband's death has prevented her from getting in touch.

Ms Liu's friend Liao Yiwu said he spoke by phone to Ms Liu earlier this week, where she said it was "easier to die than live. Using death to defy could not be any simpler for me."

Writing on the website ChinaChange, Mr Liao, a writer who now lives in Germany, said Ms Liu also told him: "I've got nothing to be afraid of. If I can't leave, I'll die in my home. Xiaobo is gone, and there's nothing in the world for me now."

Mr Liao also uploaded a recording of a phone conversation he had earlier in April with Ms Liu, where she can be heard crying for several minutes, and saying: "I'm ready to die here... if I'm dead, it'll all be done with."

Image copyright Getty Images Image caption Liu Xiaobo was branded a criminal by authorities and repeatedly jailed throughout his life

Western diplomats have called on Beijing to allow Ms Liu to travel abroad, and the German ambassador to China has told the South China Morning Post that she is welcome to go to Germany.

On Thursday, foreign ministry spokeswoman Hua Chunying said she did not have information on Ms Liu's situation.

"Liu Xia is a Chinese citizen. The relevant Chinese authorities will handle relevant issues in accordance with the law," she said.

Mr Liu, 61, was the first Nobel Peace Prize laureate to die in custody since German pacifist Carl von Ossietzky, who died in Nazi Germany in 1938.

He had been undergoing treatment for terminal liver cancer.