German and American doctors who visited sick Chinese dissident Liu Xiaobo in hospital say they believe he can be moved overseas for treatment.

In a joint statement on Sunday, the two doctors apparently contradicted statements by Chinese experts who maintain that a medical evacuation would be unsafe.

However, Joseph M Herman of the University of Texas MD Anderson Cancer Center, and Markus Buchler of the University of Heidelberg said any evacuation would have to take place "as quickly as possible".

"Liu Xiaobo and his family have requested that the remainder of his care be provided in Germany or the United States," they said.

"While a degree of risk always exists in the movement of any patient, both physicians believe Mr Liu can be safely transported with appropriate medical evacuation care and support."

READ MORE: Questions about Liu Xiaobo's treatment in China prison

The Nobel Peace Prize laureate is being treated at a hospital in the city of Shenyang for late stage liver cancer.

He was diagnosed in May while serving an 11-year sentence for inciting subversion after he helped write a petition known as Charter 08 calling for sweeping political reforms.

The experts' visit on Saturday came as Liu's illness took a turn for the worse.

Chinese doctors said they have stopped using cancer-fighting drugs so as not to overwhelm his severely weakened liver.

Beijing has come under fire from rights groups over its treatment of Liu, 61, and for waiting until he became terminally ill to release him from prison.

Shang Baojun, Liu's former lawyer and a close friend, said that during the consultation with the foreign experts, Liu was "clear-headed and communicated smoothly, and could even speak English".

"He again expressed a desire to go abroad for treatment, preferably in Germany, though the US would also be fine, and his family members said the same," Shang told The Associated Press news agency on Saturday.

"We sincerely hope this request will be approved."

In a sign of the seriousness of Liu's decline, his younger and older brothers and their wives were being allowed to see him, Shang said. At the hospital, Liu has been mostly accompanied by his wife, Liu Xia, and her brother.

A group of his friends fear he is near death and they issued an open letter earlier this week calling on the Chinese government to give them access to their ailing friend on "humanitarian" grounds.

The office of the United Nations human rights chief on Friday also expressed concern about Liu's condition and said the UN should be granted access to both Liu and his wife.

China's foreign ministry did not immediately respond to a request for comment on whether Liu would be allowed to leave.