At least 60,000 people have died in Syrian government jails during the five-year civil war, a monitoring group has said.
"No fewer than 60,000 detainees were martyred ... either as a result of direct bodily torture, or denial of food and medicine" the UK-based Syrian Observatory for Human Rights said, citing sources in the Syrian government's security apparatus.
The Observatory's director, Rami Abdulrahman, said it had arrived at the number by adding up death tolls provided by sources in several Syrian jails and security agencies.
He said more than 20,000 had died at Sednaya prison, near Damascus.
The Observatory said it had been able to verify the deaths of 14,456 people, 110 under the age of 18, since the start of the Syrian uprising in 2011.
Mr Abdulrahman said his sources were serving officials seeking to expose what was going on, and added the Observatory had been gathering the information since the start of the year.
The Observatory said its information came from "reliable sources within the regime’s security branches, most importantly the Air Force Intelligence and State Security, in addition to reliable sources in Sednaya military prison".
The Syrian government has rejected similar reports in the past.
UN investigators said in February detainees held by the Syrian government were being killed on a massive scale.
"We know large numbers of people have died in detention in Syria," Nadim Houry, deputy director of the Middle East and North Africa division at Human Rights Watch, said.
"The only way to get to the bottom of the numbers question is to allow for independent monitors into the detention centres," he added.
In 2013, a Syrian defector known as Caesar smuggled out tens of thousands of photos taken between May 2011 and August 2013 showing at least 6,786 separate individuals who had died in government custody, HRW said in a report issued in December.
The toll was calculated by the Syrian Association for Missing and Conscience Detainees (SAFMCD), which was formed by an opposition body and reviewed all the photos, the HRW report said.
President Bashar al-Assad dismissed the Caesar photos in a 2015 interview as "allegations without evidence", and part of a Qatar-funded plot against his government.
Mr Houry said: "Whether it is 60,000 or 30,000, the number is just huge. Despite the Caesar photos, the multiple reports, there is no international traction."
The UN investigators said in February the reported killings of detainees amounting to a state policy of "extermination" of the civilian population, a crime against humanity.
The independent experts said they had also documented mass executions and torture of prisoners by two jihadi groups, Isis and al-Nusra Front. These constituted war crimes and in the case of Isis also crimes against humanity.
Additional reporting by Reuters
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