US and German doctors examined China's Nobel Peace Prize laureate Liu Xiaobo on Saturday and concluded he was in a serious condition with late-stage liver cancer, the hospital treating him said.

The foreign doctors visited Liu, 61, at the hospital in the northeastern city of Shenyang following international pressure for China to let him go abroad or allow him to choose his own treatment.

Friends of Liu's fear he is near death since he was transferred from prison to the Shenyang hospital after he was diagnosed with terminal liver cancer more than a month ago.

The First Hospital of China Medical University said Liu was visited by American oncology expert Joseph Herman from the MD Anderson Cancer Center and German doctor Marcus Buchler from Heidelberg University.

The doctors, who were invited by the hospital at Liu's family's request, found that Liu was had excess abdominal fluid and was in serious condition, the hospital said on its website.

They suggested that Liu undergo an MRI to evaluate his liver's condition and decide if he should undergo radiotherapy or another type of intervention.

If his liver function improves, they could consider immunotherapy, but for now Liu will continue supportive therapy to alleviate the pain and "elevate his quality of life," the hospital said.

"The American and German specialists have fully endorsed the treatment programme and measures by the group of national experts," it said.

Patrick Poon, a China researcher from Amnesty International, welcomed the decision to allow international experts access to Liu.

But Poon said the decision "shows that China is determined not to fulfil Liu Xiaobo's wish to receive medical care abroad".

Furthermore, the experts are unlikely to be able to speak about Liu's condition, Poon said.

"Reasons like privacy could be cited as the excuse of not disclosing any details."

Beijing has come under fire from human rights groups over its treatment of the activist 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.

The doctors' visit comes as Chinese President Xi Jinping is in Hamburg, Germany, for a G20 summit ending Saturday.

Liu was sentenced to 11 years in prison in December 2009 for "subversion" after calling for democratic reform. At the Nobel ceremony in Oslo in 2010, he was represented by an empty chair.

A friend of Liu's, who asked to remain anonymous due to the case's sensitivity, told AFP that both his younger and older brothers are set to visit him in hospital for the first time this weekend.

But "Liu's friends are still not able to meet him," Poon said, which "speaks a lot about the restrictions Liu and his family face."