The brother-in-law of Chinese dissident Liu Xiaobo has denied that doctors have halted medication for him, in a letter released by the hospital treating the Nobel Peace laureate, following rumors that he was too ill for treatment to continue.

Liu, 61, was jailed for 11 years in 2009 for "inciting subversion of state power" after he helped write a petition known as "Charter 08" calling for sweeping political reforms.

He was recently moved from jail to a hospital in China's northeastern city of Shenyang to be treated for late-stage liver cancer.

"Liu Xiaobo has not stopped being medicated, but because his condition is grave and the tumor is rapidly progressing, after a consultation by top domestic specialists, the medication has been adjusted, with his family's consent," the letter read.

"I express resentment at the people who spread and create rumors and distort facts," added the handwritten document that appeared to be written and signed by the brother-in-law, Liu Hui, and was released late on Thursday on the hospital website.

A woman who answered the hospital telephone said she did not know about Liu's case.

Also on Thursday, the hospital said in a statement that doctors had stopped using sorafenib, a drug to fight liver cancer, and Chinese medicines, because of Liu's worsening condition.

Neither the statement nor the letter identified the treatments now being administered to Liu.

Earlier, the hospital had said Liu's liver function had worsened, with his levels of bilirubin gradually rising, and that he had a thrombosis on his lower left leg.

A family friend of Liu said his medication had been halted as his liver was unable to take it.

Calls have grown from rights groups, international bodies and western governments for China to allow Liu and his wife Liu Xia to be treated overseas if they wish.

The European Parliament on Thursday urged the Chinese government to immediately release the couple from house arrest, and allow Liu to seek treatment freely.

"The Chinese government often pressures family members to write statements or record videos to make claims in its favor," Patrick Poon, a China researcher for rights group Amnesty International, said in a message, referring to the letter.

"If Liu Hui is free, why can't he talk to journalists?" added Poon, who is based in Hong Kong. China's foreign ministry did not immediately respond to a request for comment on the remarks.

The hospital has invited doctors from the United States and Germany to help with Liu's treatment, the Shenyang city justice department said on Wednesday.

Diplomatic sources in Beijing say China has been nervous the concerns over Liu could overshadow President Xi Jinping's appearance at a summit of the Group of 20 nations in Hamburg, Germany, on Friday and Saturday.

At the meeting, Xi will seek to project Chinese leadership on issues such as climate change and free trade.