Two years after his Nobel peace prize, Chinese dissident Liu Xiaobo remains imprisoned, relatives are under house arrest or cowed into silence and, supporters say, the democratic change he sought seems further away than ever.

As the Nobel committee in Oslo prepares to award this year's prestigious prize on Friday, the dissident writer remains the world's only jailed Nobel peace laureate, with more than seven years left to go of a Chinese prison term for subversion.

Little is now known about Liu, 56, and his current condition -- he is said to suffer from hepatitis -- due to a curtain of silence drawn across him and his family by China's government, which was deeply embarrassed by the award and reacted angrily.

This makes it difficult to confirm whether Liu is even still at the prison in Liaoning province in northeastern China where he was initially jailed.

Liu's wife Liu Xia, remains under house arrest at their home in Beijing to prevent her speaking about her husband's case, while his brothers continue to decline media interviews for fear of losing their occasional visitation rights to him.

"I don't have any information about Liu Xiaobo and I have been unable to reach Liu Xia," Dai Qing, a fellow activist who is close to the couple, told AFP.

Liu, who was jailed previously for his involvement in the 1989 Tiananmen pro-democracy protests, was sentenced in 2009 to 11 years in jail for "attempted subversion of power" after co-authoring a bold manifesto for democracy in China.

China lashed out after his 2010 Nobel prize and refused to allow him to attend the ceremony in Oslo -- where he was represented instead by an empty chair.

Liu's supporters say his cause is in danger of being forgotten and have called for greater foreign pressure on Beijing.

"The international community must address the ongoing repression of rights in China and urge the country's current and soon-to-be leaders to... (respect) the fundamental rights of all Chinese citizens," Jared Genser, founder of the rights group Freedom Now, wrote in an essay for the Huffington Post news website.

Some have expressed faint hopes for the Lius' future, citing the fact that the ruling Communist Party will meet next month to annoint a new set of leaders whose views on reform remain a mystery.

"The release of Liu Xiaobo, or at least the end of the house arrest of Liu Xia, which, if you recall, is totally illegal, would be a way to send a signal" that China was willing to respect rights, said Jean-Philippe Beja, a French translator of Liu's writings.

But Beja, a close friend of the family who said he has been unable to reach Liu Xia, added he was "not very optimistic in the short term."

Dai said China's rights situation could indeed worsen if hardline Communist Party propaganda chief Liu Yunshan is elevated to the top echelon of power at the party congress next month.

"If that is the case, and if it is he who will oversee ideology, there will no longer be any hope. China will enter a period of darkness," she said, comparing him to Kang Sheng, an infamous former security chief under Mao Zedong.

Hu Jia, another activist who spent three years in jail on subversion charges until his release last year, said he was worried about Liu Xia's health.

He is one of the few people to have seen her recently, he said, after visiting the Liu home in eastern Beijing at night -- only to find it under heavy guard as it has been since the Nobel award.

He aborted a plan to make contact for fear of endangering her.

Hu said he believes "she is very lonely and smoking a lot", and lamented that the couple had not elicited the same outpouring of support as blind lawyer Chen Guangcheng.

Chen drew global attention in April after a daring escape from house arrest and flight to the American embassy in Beijing. He eventually was allowed to leave for the United States in an episode that severely strained China-US relations.

"Guangcheng had online supporters who would go hundreds of kilometres (miles) to see him, yet just a handful of people care to go see Liu Xia," said Hu.