A group of UN rights experts on Tuesday condemned the death in detention of a dissident Chinese activist, saying she was denied access to vitally-needed medical treatment for months.

"The death of Ms. Cao is a tragic example of the results of criminalisation of the activities of human defenders in China and reprisals against them," a group of UN rights experts said in a statement.

Cao Shunli, 52, died on Friday after being detained since last September and denied medical treatment for several months after she fell ill, her brother and lawyers acting on her behalf told AFP.

China's foreign ministry insisted on Monday that Cao's "lawful rights and interests have been protected in accordance with law," without providing further details.

The prominent human rights lawyer had campaigned since 2008 for greater government transparency, and improved access for Chinese civil society to give evidence to a periodic UN review on China's rights record.

She had been set to travel to Switzerland last September to observe a meeting about China at the UN Human Rights Council, but Chinese authorities prevented her from boarding the flight to Geneva.

She was first held incommunicado, according to the UN expert group, which includes special rapporteurs on torture and other cruel, inhuman or degrading punishment.

Her whereabouts first became known when she was charged with the "crime of provocation", they said.

One of Cao's lawyers told AFP she had fallen ill in November but had been denied access to medical treatment for more than three months. She was only sent to hospital after suffering organ failure and falling unconcious in late February.

The UN expert group said Cao's "enforced disappearance, arbitrary detention, failing health and the fact that she was denied medical care" had been brought to their attention at the time.

They had "transmitted urgent appeals to China" and issued a public statement as early as October.

China had assured them that Cao "had no serious health problems and had access to medical care".

"It is unacceptable that civil society activists pay the ultimate price for peaceful and legitimate interaction with the United Nations," the experts said, backing a call from the UN human rights office for a prompt investigation into the circumstances leading to Cao's death.

Countries including the US, Britain, France and Canada have also expressed concern at Cao's death.