By John Davison and Stephanie Nebehay
BEIRUT/GENEVA (Reuters) - The Syrian government executed up to 13,000 prisoners in mass hangings and carried out systematic torture at a military jail near Damascus, rights watchdog Amnesty International said on Tuesday.
The Syrian Justice Ministry denied the Amnesty report, calling it completely "devoid of truth", Syrian state news agency SANA reported late on Tuesday.
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Amnesty said the executions took place between 2011 and 2015, but were probably still being carried out and amounted to war crimes. It called for a further investigation by the United Nations, which produced a report last year with similar accusations also based on extensive witness testimonies.
Syria's government and President Bashar al-Assad have rejected similar reports in the past of torture and extrajudicial killings in a war that has claimed hundreds of thousands of lives.
The Amnesty report said an average of 20-50 people were hanged each week at the Sednaya military prison north of Damascus. Between 5,000 and 13,000 people were executed at Sednaya in the four years after a popular uprising descended into war, it said.
"The victims are overwhelmingly civilians who are thought to oppose the government," the report said.
"Many other detainees at Sednaya Military Prison have been killed after being repeatedly tortured and systematically deprived of food, water, medicine and medical care."
The prisoners, who included former military personnel suspected of disloyalty and people involved in unrest, underwent sham trials before military courts and were sometimes forced to make confessions under torture, Amnesty said.
SANA quoted the justice ministry as saying Amnesty's accusations were not based on real evidence but rather on "personal emotions aimed at achieving known political goals".
The ministry also accused rebel groups fighting to unseat Assad of executing and kidnapping civilians, SANA said.
The justice ministry described the report as an attempt at "harming Syria's reputation on the international stage especially after the victories of the Syrian army".
The army and allied forces drove rebel groups out of Aleppo city in December, in Assad's most important gain of the nearly six-year-old war.
The executions were carried out secretly and those killed were buried in mass graves outside the capital, with families not informed of their fate, Amnesty said.
The report was based on interviews with 84 witnesses including former guards and officials, detainees, judges and lawyers, as well as experts.
It followed a report issued a year ago by the U.N. Commission of Inquiry on Syria, whose war crimes investigators said they had documented a high number of deaths in Sednaya military prison.
"Amnesty's findings are almost completely in-line with our 'Death in Detention' paper," Paulo Pinheiro, chairman of the U.N. panel, told Reuters.
"We mentioned the executions in Sednaya and have extensive details on the systematic details of the regular ceremonies they have to conduct hangings in front of an audience of public officials. It is one of the clearest instances of a systematic practice that we had and based some of the key findings upon."
The foreign ministers of Britain and France decried Amnesty's findings. Britain's Boris Johnson tweeted: "Sickened by reports from Amnesty International on executions in Syria. Assad responsible for so many deaths and has no future as leader."
"@Amnesty has documented the horror in the prisons of the Syrian regime. This barbarity cannot be the future of Syria," said France's Jean-Marc Ayrault.
The International Committee of the Red Cross has visited selected government-run detention facilities since 2011, but its confidential findings are only shared with Syrian authorities.
"We only visit central prisons, which are under the Ministry of Interior," ICRC spokeswoman Iolanda Jaquemet said.
The ICRC has systematically requested "access to all detainees arrested by all parties to the conflict", she added.
(Reporting by John Davison in Beirut and Stephanie Nebehay in Geneva; additional reporting by Ellen Francis in Beirut; editing by Angus MacSwan and Jonathan Oatis)