Former Khmer Rouge head of state Khieu Samphan, whose ailing health has sparked concern among those who want to see him tried for crimes allegedly committed by the 1970s regime, has been released from hospital and is back in the custody of Cambodia's genocide tribunal.

"Khieu Samphan has been released from the hospital and has now been returned to the detention facility of the ECCC," Peter Foster, spokesman for the UN side of the joint tribunal, said on June 4, referring to the Extraordinary Chambers in the Courts of Cambodia.

"No further details are available at this time," he added.

The 76-year-old, who was charged with war crimes and crimes against humanity after his arrest last year, was rushed to hospital on May 21 with high blood pressure.

His lawyer and some family members say that Samphan's health has deteriorated significantly since his incarceration, and say he should be released from pre-trial detention.

Five top cadres have been arrested so far by the tribunal, including Khmer Rouge ideologue Nuon Chea, the most senior surviving leader of the 1975-79 regime that oversaw the deaths of 1.7 million people from starvation, overwork and execution as it sought to forge an agrarian utopia.

During that time, cities were emptied and their populations exiled to vast collective farms, while schools were closed and religion banned.