BEIJING (AFP) -- Fears about the health of China's cancer-stricken Nobel laureate Liu Xiaobo rose on Thursday as the hospital treating him said the liver function of the country's most prominent democracy advocate had deteriorated.

Friends voiced concerns that Liu, 61, is now near death after it emerged last month that he had been transferred from prison to a hospital due to terminal liver cancer.

Liu was sentenced to 11 years in prison in December 2009 for "subversion" after calling for democratic reform. He was awarded the Nobel in 2010, with an empty chair representing him at the ceremony in Oslo.

Beijing has come under fire from human rights groups over its treatment of Liu and for waiting until he became so ill to take him out of prison, but authorities insist he has been afforded top medical care from renowned doctors.

China has faced calls to let Liu get treatment abroad, with the US ambassador saying he would like for him to be given that option. Earlier this week, authorities said the hospital had invited US and German cancer experts to come to the country to help treat Liu at the family's request.

China Medical University No 1 Hospital, located in the northeastern city of Shenyang, said on its website that Liu's liver function "has deteriorated" and it suspects that he has a blood clot in a calf muscle.

A team of doctors led by a reputed liver cancer specialist "informed Liu's family of the latest development and Liu's family said they understood", the hospital said.

Authorities said this week Liu has received chemotherapy and traditional Chinese medicine.

Hu Jia, a prominent activist and friend of Liu, told AFP the Nobel laureate has shown new symptoms, including vomiting, kidney malfunction and the accumulation of fluids in his abdomen.

Hu still voiced hope that the authorities would let Liu go abroad, but feared they were purposefully dragging their feet out of fear he might have a chance to speak his mind before passing.

"They want to delay things until he can no longer express himself. Their greatest fear is that he'll speak," he said.

"The best medicine for Liu Xiaobo is freedom."

Friends of Liu Xiaobo and his wife, the poet Liu Xia, released an open letter requesting to be allowed to visit him on a "humanitarian basis".

"We were surprised to hear today that Liu Xiaobo's medical condition has worsened," says the letter signed by 44 scholars, writers and activists.

"We feel this is deeply tragic and realise that Liu Xiaobo has few days left and fear he is near death," it says.

"At this moment, we urgently need to go to visit him to bring to Liu Xiaobo and Liu Xia their friends' care and well wishes."

Liu's treatment has put a spotlight on China's most recent crackdown on activists, which has intensified since President Xi Jinping took office in 2013.

The deterioration of Liu's health could haunt Xi's trip to Germany, where he arrived this week to participate in a G20 summit on Friday and Saturday.

Asked about Liu's health and whether foreign doctors had arrived, foreign ministry spokesman Geng Shuang told reporters that other nations should "refrain from interfering in China's judicial sovereignty under the pretext of individual cases".

Liu was arrested in 2008 after co-writing Charter 08, a bold petition that called for the protection of basic human rights and reform of China's one-party Communist system.

He is also known for his efforts to help negotiate the safe exit from Tiananmen Square of thousands of student demonstrators on the night of June 3-4, 1989 when the military violently suppressed six weeks of protests in the heart of Beijing.

Describing the Chinese government's treatment of Liu as "inhuman", Amnesty International's China researcher Patrick Poon said the Nobel laureate had "less and less hope of receiving better medical treatment".

"All this is suggesting that he seems to be in his last days now," Poon, who has spoken to friends of Liu, told AFP. "The Chinese government wants him to die in China."

Human Rights Watch's China director, Sophie Richardson, voiced concern about what will happen to Liu Xia after he husband dies.

The poet has been under house arrest since 2010 but she was allowed to be with her husband at the hospital.

"Independent of what happens to him, the authorities have made clear their intention to silence and torture her too," Richardson told AFP. "They won't want her to speak freely."