The United States has said it is "deeply disturbed" by reports of the death of activist Cao Shunli at a hospital in Beijing.
Cao's lawyers said she had died in detention after being denied medical treatment for several months.
The human rights activist was arrested at Beijing airport in September, when she attempted to board a flight to Geneva to attend a UN training course.
She had previously served at least two terms in prison camps for her activism.
Human Rights Watch said that Cao was only transferred to hospital in February, despite her medical condition worsening in her months in detention.
Cao had advocated for the right of independent civilians to participate in the Chinese government's drafting of human rights reports.
US State Department spokeswoman Jen Psaki said that the US had repeatedly raised concerns about Cao Shunli's detention and deteriorating health with the Chinese authorities.