The Syrian regime burned the bodies of “thousands” of prisoners in a giant crematorium at its most notorious jail in an effort to hide the scale of its killings, the top US diplomat for the Middle East revealed Monday.

The bodies of detainees were incinerated next to Sednaya prison outside the capital Damascus in order to "manage" the numbers and destroy the evidence, Stuart Jones, acting assistant secretary for the State Department Bureau of Near Eastern Affairs, told reporters.

"We believe that the building of a crematorium is an effort to cover up the extent of mass murders taking place in Sednaya prison," Mr Jones said.