Syrian government 'systematically rejected' requests to visit prisons accused of torturing inmates, UN says

Thousands of prisoners are believed to have been subjected to cruel treatment.

Thousands of prisoners are believed to be subjected to cruel treatment, including torture according to The Associated Press.

"We are extremely concerned that thousands of civilians continue to be held in government detention facilities and have grounds to believe that they are systematically subjected to cruel, inhuman, and degrading treatment including torture and sexual violence," Dujarric said.

The U.N. could not independently verify U.S. reports that thousands of people were killed under Bashar al-Assad's regime from 2011 to 2015 and that a crematorium was used to to dispose of their bodies, Dujarric said.

The executions and cremations are believed to have taken place at the Saydnaya prison complex.

Assad's government has denied the allegations made by the U.S.

ABC News' Connor Finnegan, Kirit Radia and The Associated Press contributed to this report.