The widow of Chinese Nobel Peace Prize-winner Liu Xiaobo has appeared for the first time since her husband's funeral in a video posted on YouTube, which is blocked in China.

Liu Xia's friends have raised concern about her fate, saying they have not been able to speak to her since her husband's sea burial on July 15.

She was last seen in government-released images of Liu Xiaobo's funeral.

"I am recovering in a province outside of Beijing. I ask you to give me time to mourn," said Liu in the minute-long video posted on Friday.

Dressed in a black t-shirt and black trousers, Liu Xia was sitting on a white sofa next to a coffee table while holding a lit cigarette.

"I will see you one day in top form. While Xiaobo was sick, he also looked at life and death with some distance, so I also have to readjust. I will be with you again when my situation generally improves," she said.

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Liu Xia, 56, has been under effective house arrest since her husband, a prominent dissident since the 1989 Tiananmen Square pro-democracy protests, won the Nobel Prize in 2010. He was sentenced to 11 years in jail on subversion charges in 2009.

Friends of the couple raised questions about whether Liu Xia made the comments in Friday's video out of her own free will.

"It is certain that she was forced by the authorities to make this video," Hu Jia, a Chinese dissident and friend of the couple, told the AFP news agency on Saturday.

"How can anyone who does not even enjoy freedom express her will freely?"

The name of the filmmaker, the place and date of filming, were not specified, but it would be unusual for the video to be released without the knowledge of the authorities. plain-clothes security agents guard Liu Xia's Beijing apartment.

Jared Genser, Liu Xia's lawyer, who has filed a complaint to the United Nations, has accused the Chinese government of her "enforced disappearance".

Chinese authorities have said that Liu Xia was free and have told diplomats who asked about her whereabouts that her lack of communication was due to her desire to mourn in peace.

Before the death of her husband, Liu Xia had told diplomats and friends that she wished to leave China should Liu Xiaobo be released.

Liu had requested to receive treatment abroad after his terminal cancer diagnosis, a wish that friends believe was in reality for his wife's sake. The government, however, refused to release him.

He died aged 61 while still in custody at a Chinese hospital on July 13, becoming the first Nobel Peace Prize winner to die in custody since German pacifist Carl von Ossietzky in 1938.