At least 60,000 people have died in Syrian government prisons over the past five years from torture or due to dire humanitarian conditions, including a lack of food, a monitor said Saturday.

The head of the Syrian Observatory for Human Rights monitoring group, Rami Abdel Rahman, said he compiled the toll from regime sources.

"Since March 2011, at least 60,000 people lost their lives to torture or to horrible conditions, notably the lack of medication or food, in regime detention centres," Abdel Rahman told AFP.

He said the highest number of deaths had been recorded in the infamous Saydnaya prison as well as detention centres run by Syria's notorious air force intelligence and state security forces.

Thousands of prisoners are held in the military-run Saydnaya prison, one of the country's largest detention centre located 30 kilometres (18 miles) north of Damascus.

Rights groups have accused Syria's government of systematically using torture and inhumane practices in its detention centres.

A UN probe in February accused the Syrian government of a policy of "extermination" in its jails.

The Britain-based Observatory says it has compiled a list of 14,456 names -- including 110 children -- who have died in regime prisons.

According to Abdel Rahman, government forces have arrested a total of 500,000 people since Syria's conflict erupted in 2011.

While some have been released and others died, the whereabouts of thousands of detainees remain unknown.

Abdel Rahman also said that "several thousand people" have died while being held by rebel groups and jihadist factions like the Islamic State group.

In early 2014, a regime defector calling himself "Caesar" smuggled out of Syria some 55,000 photographs depicting the tortured and abused bodies of around 11,000 people who had reportedly died in Syrian jails during the first two years of the conflict.

Earlier this month, the UN special envoy to Syria Staffan de Mistura named Eva Svoboda to oversee progress on the issue of detainees.