Syrian intelligence agencies are running a network of torture centers across the country where detainees are beaten with batons and cables, burned with acid and sexually assaulted, Human Rights Watch said in a report on Tuesday.
The state-sanctioned abuses amounted to crimes against humanity that should be investigated by the International Criminal Court, the New York-based campaign group added.
The report identified 27 detention centers it said intelligence agencies have been using since March 2011m when President Bashar Assad's government began a crackdown on pro-democracy protests that have grown into an armed revolt.
Syria's government did not immediately respond to the accusations which have been echoed in earlier reports by the United Nations.
Human Rights Watch said tens of thousands of people had been detained by Syria's Department of Military Intelligence, the Political Security Directorate, the General Intelligence Directorate and the Air Force Intelligence Directorate.
The group said it conducted more than 200 interviews with people who said they were tortured, including a 31-year-old man who was detained in the Idlib area in June and made to undress.
"Then they started squeezing my fingers with pliers. They put staples in my fingers, chest and ears. I was only allowed to take them out if I spoke. The staples in the ears were the most painful," he was quoted as saying.
"They used two wires hooked up to a car battery to give me electric shocks. They used electric stun-guns on my genitals twice. I thought I would never see my family again. They tortured me like this three times over three days," he added.
Human Rights Watch said it documented more than 20 torture methods that "clearly point to a state policy of torture and ill-treatment and therefore constitute a crime against humanity".
United Nations human rights investigators have made similar reports.
"Torture is one of the most extensively and best documented of the many awful human rights violations taking place in Syria over the past 15 months," U.N. human rights spokesman Rupert Colville told a news briefing in Geneva on Tuesday.
There has been a "constant stream of very consistent information about widespread systematic use of torture, and of course if it is widespread and systematic, that amounts to a crime against humanity," he said.
A recent U.N. mission to the region interviewed people who had faced severe beatings, electric shocks, cigarette burns, mock executions, and sleep deprivation, he said.
Human Rights Watch called for the U.N. Security Council to refer Syria to the International Criminal Court (ICC) and impose sanctions on officials carrying out abuse.
"The reach and inhumanity of this network of torture centers are truly horrific," Ole Solvang, emergencies researcher at Human Rights Watch said. "Russia should not be holding its protective hand over the people who are responsible for this."
Russia - an ally of Syria - and China have already vetoed two council resolutions that condemned Damascus and threatened it with sanctions.
The United Nations has said more than 10,000 people have been killed during the 16-month Syria conflict.
Meanwhile, The Syrian army pressed an offensive against rebels on Tuesday, bombarding the city of Douma near Damascus, and Turkey said it had scrambled F-16 fighters the previous day after Syrian helicopters flew near its border.
Turkey's armed forces command said the jets took off on Monday when Syrian transport helicopters were spotted flying near the frontier, without entering Turkish air space. It was the third day in a row that Turkey had scrambled its F-16s.
Syrian President Bashar al-Assad told a Turkish daily he wished his forces had not shot down a Turkish jet last month and that he would not allow tensions with Turkey to lead to war.
The downing of the Turkish F4 in disputed circumstances has aggravated hostility between Damascus and Ankara, which has raised its military profile to try to keep Syrian helicopters from Turkey's border zone where rebels and refugees are camped.
"We learned it belonged to Turkey after shooting it down. I say 100 percent 'if only we had not shot it down'," Turkey's Cumhuriyet daily quoted Assad as saying.
A Syrian general, as well as 84 soldiers were the latest to defect and flee to Turkey on Monday. But army and government defections have so far failed to shake Assad's 12-year grip on power.
Assad told Cumhuriyet he was not bent on staying in office come what may, but gave no hint that he was ready to quit.
"If my staying or going saved my people and country, why would I hold on? I wouldn't even stay one day," he said.
"If the opposite is true, that is, if the people don't want me, then there are in any case elections. If the people wanted, they would send me away," Assad was quoted as saying.
The complete Human Rights Watch report, which includes maps of the detention centers, can be seen here: http://www.hrw.org/reports/2012/07/03/torture-archipelago-0
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