Image copyright Reuters Image caption Liu Xiaobo is a poet and human rights campaigner

Beijing has hit back at Washington for "irresponsible remarks" after the US criticised its treatment of Chinese Nobel peace laureate Liu Xiaobo.

The political activist, who is serving an 11 year term on subversion charges for calling for greater democracy, has been moved to hospital after being diagnosed with terminal liver cancer.

His wife Liu Xia, who is under house arrest, says it is beyond treatment.

The US has called on China to give the couple "genuine freedom".

"We call on the Chinese authorities to not only release Mr Liu but also to allow his wife Ms Liu Xia out of house arrest," US embassy spokeswoman Mary Beth Polley said.

Some American politicians have also called on China to allow the dissident, a key leader in the Tiananmen Square protests in 1989, to travel overseas for medical help.

But Chinese foreign spokesman Lu Kang criticised the calls, saying: "China is a country with rule of law. Everyone is equal before the law. All other countries should respect China's judicial independence and sovereignty and should not use any so-called individual case to interfere in China's internal affairs."

Mr Liu was awarded the Nobel Peace Prize in 2010, with the Nobel committee describing the jailed dissident as "the foremost symbol" of the human rights struggle in China.

He was not allowed to collect his prize and was represented at the awards ceremony by an empty chair. The Chinese government, which regards him as a criminal, was infuriated by the award.

Following the award, Mrs Liu was placed under house arrest, although she was never charged with a crime. The Chinese authorities have never explained why they restricted her movements.

According to friends, Mrs Liu has been allowed to visit her husband, who is being treated in hospital in northern Liaoning Province after being diagnosed a month ago.

Image copyright Reuters Image caption Protesters placed photos of Mr Liu and his wife Xia outside China's Liaison House Office in Hong Kong

The Nobel laureate was diagnosed with cancer on 23 May, lawyer Mo Shaoping told the South China Morning Post. He was released days later and is now being treated in the northern city of Shenyang.

"He has no special plans. He is just receiving medical treatment for his illness," Mr Mo told AFP news agency.

But speaking in a video which was shared online this week, a tearful Mrs Liu said: "[They] cannot perform surgery, cannot perform radiotherapy, cannot perform chemotherapy."

A statement from the government in Liaoning said Mr Liu had been released on medical parole and was being treated by eight tumour experts.

Image copyright AFP Image caption Liu Xia (seen with lawyer Mo Shaoping, left) has been allowed only brief, intermittent visits to her husband

Mr Liu has three years left to serve of an 11-year sentence for "inciting subversion" after drafting Charter 08 - which called for multi-party democracy and respect for human rights in China.

Amnesty International said he should never have been jailed.

It urged China to ensure he received "adequate medical care, effective access to his family and that he and all others imprisoned solely for exercising their human rights are immediately and unconditionally released".

Following his Nobel award, China froze diplomatic ties with Norway - relations were normalised only last December.