DUBAI, United Arab Emirates — Human rights groups say an activist in the Arab Gulf state of Oman has died while serving a three-year prison sentence on charges related to his online activism.

A coalition of four rights groups, including the Omani Association for Human Rights and the Gulf Center for Human Rights, said Wednesday that Hassan al-Basham passed away on April 28 in Samail Central Prison after a sharp drop in his blood sugar levels.

The rights groups say his deteriorating health was known to the courts, and that a request by his lawyers for a medical examination was ignored by an appellate court.

The rights groups say he'd been detained since September 2015 and was convicted on charges including insulting Oman's ruler and using the internet to prejudice religious values.