Duch, 76, was given a life sentence for crimes against humanity after first trial at UN-backed Khmer Rouge Tribunal.

A Khmer Rouge-era prison chief who was the first person to be convicted in an ongoing Cambodian war crimes tribunal has been admitted to hospital in Phnom Penh with respiratory problems.

Kaing Guek Eav, better known as Duch, was sentenced to life in jail in 2012 for his role in running the S-21 detention centre where thousands were incarcerated, tortured, and sent to their deaths in the nearby “killing fields”.

Chat Sineang, director of the Kandal prison where Duch has been serving his sentence, told the AFP news agency the 76-year-old has been undergoing treatment for respiratory problems since October 20 and was moved from a provincial clinic to the capital.

“He is still at the Russian Friendship hospital,” Sineang said, describing Duch’s condition as “quite serious because he is 76 now”.

S-21 was at the heart of the security apparatus of the Khmer Rouge, an ultra-Maoist regime that ruled Cambodia from 1975 until 1979 when an estimated 1.7 million people were killed or died from starvation, exhaustion or disease.

The UN-backed tribunal was set up in 2006, but has been hampered by a lack of funds and allegations of political interference.

Duch, a former mathematics teacher turned torturer, was arrested in 1999 after being discovered working for a Christian aid agency under a false name.

His eventual trial was followed closely and the conviction seen as a milestone for victims seeking justice.

Judges at the tribunal are set to announce their verdict in the court’s second case next month.

Former leaders Nuon Chea and Khieu Samphan are on trial for crimes against humanity in what may be the last case the court hears. Both men are already serving life sentences after being found guilty in an earlier trial.

Age has also played a role in thwarting the tribunal’s work.

Ieng Sary – a former Khmer Rouge foreign affairs minister who had been charged with crimes against humanity – died in 2013 before the court could reach a verdict in his case.

His wife and co-defendant Ieng Thirith, a former social affairs minister, was released because of dementia and died in 2015.