Beirut: Syrian President Bashar al-Assad's government has secretly executed between 5000 and 13,000 people in just one prison as part of its campaign to eliminate opposition to his rule, a new report by the watchdog group Amnesty International has found.

The killings took place over a four-year period between 2011 and 2015 in the notorious Sednaya facility outside Damascus, and the bodies were later disposed of in mass graves, according to the report released on Monday by Amnesty.

Syrian prisoners sit in a courtroom before their release in Damascus in 2012, while up to 13,000 others have been executed since then. Credit:AP

Human rights groups estimate that tens of thousands of political prisoners have disappeared in the Syrian prison system since the uprising against Assad's rule first erupted in 2011, and they suspect many of those have been tortured to death or secretly killed.

The accounts of these killings are in addition to the figure of 17,000 that Amnesty counted in an earlier report on the extra-judicial killings issued in August 2016, compounding an emerging picture of what Amnesty referred to as a policy of "extermination" against opponents of the government.