The Bashar al-Assad government in Syria has committed extermination of the detainees in detention centres and war crimes, the United Nations Human Rights Council (UNHRC) said in a report, according to BBC.

UNHRC's Independent Commission of Inquiry's submitted the report titled "Out of Sight, Out of Mind: Deaths in Detention in the Syrian Arab Republic," with findings based on 621 interviews. The study consists of crimes committed against thousands of detainees between March 10, 2011, and Nov. 30, 2015.

"In the Syrian Arab Republic, massive and systematised violence – including the killing of detainees in official and makeshift detention centres – has taken place out of sight, far from the battlefield," the study read.

"... civilians, mainly males above the age of 15, were arbitrarily arrested and detained by the Syrian security and armed forces or by militia acting on behalf of the Government during mass arrests, house searches, at checkpoints and in hospitals," it added.

The report suggests that the detainees held by Syrian government forces were "beaten to death, or died as a result of injuries sustained due to torture." Majority of the injured detainees were left without medical care.

"Others perished as a consequence of inhuman living conditions. The Government has committed the crimes against humanity of extermination, murder, rape or other forms of sexual violence, torture, imprisonment, enforced disappearance and other inhuman acts. Based on the same conduct, war crimes have also been committed," the report said.

According to the report, "Government authorities administering prisons and detention centres were aware that deaths on a massive scale were occurring."

Investigators found that people continue to die in prisons and these incidents remain unnoticed.

"Deaths in custody continue to occur in near-total secrecy and are largely lost to the international public and political discourse surrounding the violence of the conflict, in spite of their devastating impact on the lives of hundreds of thousands of Syrians," the report read.

They said Damascus needs to take immediate measures to put an end to torture and deaths of civilians and others detained in prisons and other detention centres.

"The situation of detainees in Syria is critical, and represents an urgent and large- scale crisis of human rights protection. With thousands of persons still in custody, urgent steps need to be taken by the Syrian Government, armed groups, the external backers of various belligerents, and the wider international community to prevent further deaths."