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.

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.