A Chinese activist allegedly detained to stop her taking part in a UN review of the country's rights record is seriously ill in custody, her lawyer and an advocacy group said Wednesday.

Police detained Cao Shunli last September at Beijing's international airport, her lawyer Wang Yu said, adding that she had been travelling to testify at a UN Human Rights Council review in Switzerland.

Cao became seriously ill while being held and her condition was worsened by an earlier denial by police of medical care, Wang said, adding that her condition has improved slightly since Tuesday.

"Although the infection has been reduced, she is still in danger," Wang said, citing information from Cao's brother. "I am still very worried about her."

Advocacy group Chinese Human Rights Defenders cited one of Cao's close friends as saying: "She got a little better. Her fever is gone now. She opened up her eyes after they pumped the fluid out of her abdomen."

The US-based group earlier said she had fallen unconscious after suffering organ failure late last month.

Cao was said by rights groups including Amnesty International to be one of a group of activists who held a sit-in protest outside China's forfeign ministry in June to demand greater participation in the UN's review of China.

Amnesty called her a "prisoner of conscience" and said she should be released "immediately and unconditionally".

Beijing police were not immediately available for comment on Cao's case.

China won a seat on the UN Human Rights Council in November along with Russia, Saudi Arabia and Cuba, despite fierce international criticism of their records.

European countries and the US regularly condemn China on human rights, citing cases including its jailing of dissidents and treatment of ethnic minorities.

China for its part insists that its citizens enjoy ever-increasing freedoms as a result of growing prosperity in recent decades.