The United Nations human rights office expressed horror Tuesday at a report accusing the Syrian regime of killing and torture on an "industrial scale", saying the allegations must be investigated.
"This report is extremely alarming, and the alleged scale of the deaths in detention, if verified, is truly horrifying," Rupert Colville, spokesman for UN rights chief Navi Pillay, told AFP in an email.
"Allegations this serious cannot be ignored and further investigation is clearly necessary," he added.
The report by three international prosecutors -- commissioned by opposition-backer Qatar and based on some 55,000 digital images from a defector -- charges that President Bashar al-Assad's regime is guilty of the "industrial scale" torture and killing of 11,000 detainees.
The report, which was first released in The Guardian newspaper, CNN and Turkey's Anatolia news agency, shows evidence of starvation, strangulation and beatings, and features pictures of emaciated corpses with livid wounds.
It was released a day before a long-awaited international peace conference aimed at negotiating an end to Syria's bloody civil war was due to begin in the Swiss town of Montreux.
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Syria has previously denied torturing detainees but the government had no immediate reaction to the report.
Both Pillay's office and an independent commission of inquiry on Syria, appointed by the UN Human Rights Council in Geneva in 2011, have already documented a number of cases of torture similar to those described in the report, but not on such a large scale.
The four-member commission, which includes former war crimes prosecutor Carla del Ponte, has repeatedly accused the Syrian regime of crimes against humanity and war crimes.
It has never gained access to Syria, but through more than 2,000 interviews in the surrounding region or by phone or Skype it concluded that regime forces have applied torture, including electric shocks, cigarette burns, sleep deprivation and mock executions, Colville pointed out.
The UN investigators, who found that reported deaths in Syrian custody rose markedly in 2013, have also accused the opposition of war crimes, and Pillay expressed deep concern last week that some anti-government armed groups were increasingly torturing and murdering people in their custody.
The report released Tuesday "underlines the importance of independent monitors ... being allowed unfettered access immediately to Syria," Colville said.