A Chinese medical team treating imprisoned Nobel Peace laureate Liu Xiaobo has stopped using cancer-fighting drugs on him, raising concerns that China's most prominent political prisoner is critically ill.

They want to prevent chances of overwhelming his severely weakened liver.

The team decided to stop the use of an inhibitor drug for advanced liver cancer in light of Liu's rapidly deteriorating liver functions, according to a statement on Friday on the website of the First Hospital of China Medical University.

It said a traditional Chinese medicine anti-tumor treatment was also suspended while low-molecular heparin had been added to treat venous thrombosis developing in Liu's left leg.

The latest statement appears to be part of an effort by Beijing to show it is providing Liu with the best possible care, amid questions about his prison conditions and international calls for him to be freed to seek treatment abroad.

Zeng Jinyan, a close family friend in contact with Liu's brother-in-law Liu Hui, confirmed on Friday that Liu's situation was not optimistic.

Citing Liu Hui, Zeng said the inhibitor drug, Sorafenib, has failed to work on Liu Xiaobo.

"He is yet to see any improvement after 2 to 3 weeks, but its side effects are causing his liver functions to badly deteriorate with severe accumulation of abdominal fluid," Zeng wrote in a statement posted online.

"So the drug must be suspended, and the focus has shifted to preserve his liver and to give his body a chance to breathe."

Liu was diagnosed with late-stage liver cancer in May while serving an 11-year sentence for inciting subversion by advocating sweeping political reforms that would end China's one-party rule.

He was awarded the Nobel Peace Prize in 2010, the year after he was convicted and jailed by a Chinese court.