The Syrian government carried out a policy of extermination against thousands of detainees at prisons across the country, say investigators from the UN Human Rights Council.

In the report, entitled Out of Sight: Out of Mind: Deaths in Detention in the Syrian Arab Republic, investigators accuse President Bashar al-Assad’s regime of crimes against humanity and war crimes.

The study says that detainees were routinely tortured, with some beaten to death while others were left to die from malnutrition, dehydration, or lack of basic medical care.

The report also accuses various rebel groups of committing possible war crimes.

In uncovering evidence for their report, investigators interviewed hundreds of witnesses and others with information about the actions of the various pro-regime and rebel groups in the Syrian conflict.

Government and rebel forces are accused of using violence against their detainees, but the vast majority of those in captivity are being held by regime forces in a large network of prisons.

Former detainees described inhumane conditions, where officers ordered their subordinates to carry out specific methods of torture against those under their guard. The bodies of detainees who died in detention were carried through the corridors in front of other prisoners, and we often left in toilets for storage before being transported and dumped in mass graves.

Thousands of detainees are believed to have died since the protesters began in March 2011, when government forces began rounding up peaceful demonstrators and sparked an armed uprising.

In a statement, UN human rights investigator Sergio Pinheiro said:

“Government officials intentionally maintained such poor conditions of detention for prisoners as to have been life-threatening, and were aware that mass deaths of detainees would result.

“These actions, in pursuance of a state policy, amount to extermination as a crime against humanity.”

The report also accuses the Islamic State of illegally holding large numbers of prisoners, with torture and execution a routine part of their so-called system of justice.

The conflict has claimed the lives of more than 250,000 people to date, and forced 4.6 million people to flee Syria. A further 13.5 million people are considered in desperate need of humanitarian care within the country.