The Syrian government has "systematically rejected" multiple requests to visit prisons suspected of torturing inmates, a United Nations spokesperson said Tuesday.
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Thousands of prisoners are believed to be subjected to cruel treatment, including torture according to The Associated Press.
"Various U.N. entities have regularly documented and reported on human rights violations in Syria, including torture in the context of detention," spokesman Stephane Dujarric told reporters.
"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.
"Although the regime's atrocities are well documented, we believe that the building of a crematorium is an effort to cover up the extent of mass murders taking place in Saydnaya prison," said Stuart Jones, the acting assistant secretary for Near Eastern affairs. "We are appalled by the atrocities that have been carried out by the Syrian regime, and these atrocities have been carried out seemingly with the unconditional support of Russia and Iran."
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.