Last month, Human Rights in China quoted Cao Shunli's lawyer as saying she suffered from tuberculosis, liver disease and other conditions and was not receiving proper medical treatment.
China's Foreign Ministry has denied that she died because of a lack of medical care.
UN deputy spokesman Farhan Haq said yesterday that Ban shares the "deep concern" of human rights chief Navi Pillay that Cao was detained for seeking to allow the public to contribute to a report reviewing human rights in China for the UN and saddened at her death Friday.