Syrian President Bashar al-Assad's regime is following a policy of torturing and mistreating detainees in what amounts to a crime against humanity, Human Rights Watch (HRW) said in a report.

The multimedia report details the methods of torture used by the intelligence agencies at 27 detention centres that have been identified so far across the country.

The organisation conducted more than 200 interviews with former detainees and defectors since the uprising began in March 2011.

"The intelligence agencies are running an archipelago of torture centres scattered across the country," HRW researcher Ole Solvang said.

"By publishing their locations, describing the torture methods, and identifying those in charge we are putting those responsible on notice that they will have to answer for these horrific crimes," Solvang added.

In its report, Torture Archipelago, HRW cited a detainee's account of the horrific methods of torture to which he was subjected. Elias, one of the detainees who was held and torture at branch 285 of the department of general intelligence in Damacus, said: "The guards hung me by my wrists from the ceiling for eight days. After a few days of hanging, being denied sleep, it felt like my brain stopped working. I was imagining things. My feet got swollen on the third day.

"I felt pain that I have never felt in my entire life. It was excruciating. I screamed that I needed to go to a hospital, but the guards just laughed at me."

The methods employed in the state sanctioned torture included beatings with batons and cables, holding detainees in painful stress positions for prolonged periods, as well as electric shocks, acid burns, sexual assaults and humiliation. Detainees also had their fingernails torn out and were forced to undergo mock executions.

A 13-year-old boy described receiving electric shocks at the security branch near Tal Kalakh, where he was interrogated for three days. "They said, 'You pigs, you want freedom?' They interrogated me by myself," he related. "They electrocuted me on my stomach, with a rod. I fell unconscious.

"When they interrogated me the second time, they beat me and electrocuted me again. The third time they had some pliers and they pulled out my toenail."

HRS has called for the UN security council to refer Syria to the International Criminal Court at The Hague and to adopt targeted sanctions against those responsible for carrying out abuse in the regime's crackdown on pro-democracy protesters.

Commenting on the report, foreign secretary William Hague said it should act as clear warning.

"Those responsible for systematic and widespread human rights violations should not delude themselves: we and our international partners will do everything we can to ensure that they will face justice," the BBC quoted Hague as saying.