Updated at 11:20 a.m. ET Tuesday

The U.S. State Department laid out a new case against Syrian President Bashar Assad's regime on Monday: Not only has the Syrian government committed mass atrocities at its military prison complex outside Damascus, but for years, it has also added to the structure in order to burn and secretly dispose of thousands of its victims' remains.

"Beginning in 2013, the Syrian regime modified a building within the Saydnaya complex to support what we believe is a crematorium," Stuart Jones, acting assistant secretary for Near East affairs, told reporters at a special media briefing, circulating satellite photographs that he says depict that crematorium.

"Although the regime's many atrocities are well-documented," Jones continued, "we believe that the building of a crematorium is an effort to cover up the extent of mass murders taking place in Saydnaya prison."

Citing "credible sources" and data gathered by nongovernmental organizations, Jones stated that between 2011 and 2015, the Assad regime abducted and detained as many as 117,000 people and killed as many as 50 detainees a day at Saydnaya.

An Amnesty International report, titled "Human Slaughterhouse," detailed the conditions behind these numbers, describing a setting where many detainees "have been killed after being repeatedly tortured and systematically deprived of food, water, medicine and medical care."

Omar al-Shogre, who says he spent 10 months at Saydnaya, told NPR's Robert Siegel that this physical torture was compounded by unimaginable psychological torture:

"We were requested by jailers to kill each other. Sometimes the wardens came with a knife or a rope, and they asked prisoners whether they had relatives or friends in prison."And once they identified their friends and relatives, they gave them one of two options: either killing their relatives or being killed themselves. And in many cases, being killed themselves included also being tortured before being killed."

The Syrian regime dismissed the accounts included in Amnesty International's report as "not based on correct evidence but on personal emotions that aim to achieve well-known political goals."

On Tuesday, after this post published, Syria's state-run news agency SANA quoted an unnamed source at the country's Foreign Ministry who pushed back against Jones' account. U.S. allegations of a crematorium are "a new Hollywood story" that is "fundamentally untrue," the source said, accusing U.S. intelligence of having fabricated other claims against Syria in the past.

In his briefing, Jones said the alleged crematorium "could dispose of detainees' remains with little evidence" once they had been hanged or otherwise killed.

Though Jones spoke to U.S. media about the Syrian government, his message Monday did not appear directed at either party so much as at a pair of other countries: Syrian allies Russia and Iran.

"The United States is on record, has stated many times, that we are appalled by the atrocities that have been carried out by the Syrian regime," Jones said. "And these atrocities have been carried out seemingly with the unconditional support from Russia and Iran."

The Trump administration hit a Syrian air base with a missile strike last month in retaliation for the regime's deadly chemical attack in Idlib province — a unilateral strike that prompted a skirmish of words between U.S. and Russian officials.

"My assessment is the U.S.-Russia relationship is at an all-time low, the lowest point it has been since the end of the Cold War," Secretary of State Rex Tillerson told NPR several weeks after the strike. "And I would tell you that [Russian officials'] response was they didn't disagree with that."

"Russia has either aided in or passively looked away as the regime has conducted an airstrike against a U.N. convoy, destroyed east Aleppo and used chemical weapons, including sarin, against civilians in Idlib province on April 4," Jones elaborated Monday.

He also expressed doubts about a recent Russia-brokered deal to "de-escalate" the 6-year-old Syrian civil war, which bore Russia, Iran and Turkey as guarantors. "In light of the failures of the past cease-fire agreements, we have reason to be skeptical," he said.

Still, Jones said the State Department has not presented its evidence of the alleged crematorium directly to Russia — and he said it is not prepared to signal much else publicly.

Copyright 2017 NPR. To see more, visit http://www.npr.org/.