Concern is growing for a pastor imprisoned in Vietnam over charges linked to his campaigns for greater freedom of religion in the country.

According to religious freedom charity Christian Solidarity Worldwide (CSW), Pastor Nguyen Trung Ton is suffering from long term effects of COVID-19 as well as an eye disease which has caused near total loss of vision.

Local reports claim authorities at Gia Trung Prison are refusing to allow him to be examined or treated while his family are unable to send him medicine.

There's further concern over conditions at the prison with suggestions of a lack of clean water with his wife now said to be worried for his life.

CSW’s founder president Mervyn Thomas said: "We are deeply concerned for Pastor Nguyen Trung Ton’s health and call on the Vietnamese government to ensure that he has access to the urgent medical attention that he requires.

"We urge the authorities to immediately release him and other human rights defenders detained unjustly, including Nguyen Bac Truyen, Pham Van Troi and Truong Minh Duc, all of whom stood trial on the same day as Pastor Ton in April 2018."

Pastor Ton has been a vocal advocate for the right to freedom of religion or belief and spoken out against social injustices. He was sentenced to 12 years in prison in 2018 on charges of "carrying out activities aimed at overthrowing the government".