The international experts also decried crimes of Jihadist groups such as the "Islamic State" (IS) and the al Qaeda affiliated Nusra Front at the press conference on Monday.

Various factions have killed thousands of prisoners since the Syrian civil war started in March 2011, according to the UN investigators.

However, the situation was particularly grim in the detention centers controlled by the regime, the panel said.

"The mass scale of deaths of detainees suggests that the government of Syria is responsible for acts that amount to extermination as a crime against humanity," commission head Paulo Pinheiro told reporters in Geneva.

The UN report, titled "Out of Sight, Out of Mind: Deaths in Detention," is based on over six hundred interviews examining killing and torture, and includes the accounts of former prisoners.

"Nearly every surviving detainee has emerged from custody having suffered unimaginable abuses," Pinheiro said.

Officials 'were aware' of deaths

According to one witness, an elderly man was beaten up and hung from his wrists in a military facility in Homs.

"The guards burned his eyes with a cigarette and pierced his body with a heated, sharp metal object," the report said, adding that "after hanging in the same position for three hours, the man died."

Most of the victims were male civilians, Pinheiro said.

"Prison officials, their superiors throughout the hierarchy, high-ranking officials in military hospitals and the military police corps as well as government were aware that deaths on a massive scale were occurring," Pinheiro added.

Despite this, they "did not take action to prevent abuse, investigate allegations or prosecute those responsible," the report stated.

'Total impunity'

Commission member Carla del Ponte urged the UN Security Council to prosecute the crimes in Syria in the International Criminal Court.

"It depends on the political will of states. Apparently for Syria now, there is none - there is total impunity, unfortunately," she said.

The UN commission also called on the Security Council to impose "targeted sanctions" on high-ranking Syrian officials connected to deaths, torture and disappearances in custody, but stopped short of naming the suspects.

Prisons 'full of insects'

Last week, activist Raneem Matouq spoke about her two-month long imprisonment at Military Security Damascus Branch 227 in 2014.

"I was with 10 other girls in a room one-and-a-half meters (five feet) long by two meters long. For guys it was a room the same scale but they had 30-40 men, with dead bodies," Matouq told Reuters news agency in Geneva.

"It was full of insects, we were sleeping on the floor, there was no toilet," she said. "We were allowed to go to the toilet three times a day, we called it 'the picnic' because we could walk outside.

"Sometimes we would find dead bodies inside the toilet (area). It was so horrible, they were all men," she added.

dj/es (AFP, Reuters)