Acting US Assistant Secretary for Near Eastern Affairs Stuart Jones said the Assad government "has sunk to a new level of depravity" with the support of Russian Federation and Iran.
"We believe that the building of a crematorium is an effort to cover up the extent of mass murders taking place in Sednaya prison", he said in a briefing for reporters.
Though Jones spoke to USA 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 Russian Federation and Iran.
In light of the information about the mass killings, Jones said that Russia's calls for "de-escalation zones" in the country would be met with "skepticism". They were pictured shaking hands and broadly smiling with President Donald Trump before an Oval Office meeting in which discussions centered on Syria.
Jones showed a series of aerial photos taken from 2013 to 2017 that he said showed the construction of a crematorium beside the infamous prison. Assessment of the facility, he said, included the presence of "the discharge stack, the probable firewall, the probable air intake - this is in the construction phase - this would be consistent if they were building a crematorium".
The official said 400,000 people, mostly civilians, had died in the Syrian civil war so far.
"We're not going to signal what we're going to do and what we're not going to do", Jones said, but he added that the United States would "bring evidence forward to the global community".
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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.
And he said he hoped to work with Putin, whose forces are in Syria to protect Assad's regime, in this goal. "And Russia must bear responsibility to ensure regime compliance". The non-binding measure, which passed 392-3, denounced a litany of crimes perpetrated by the Assad regime, including having committed "widespread torture and rape, employed starvation as a weapon of war, and massacred civilians, including through the use of chemical weapons, cluster munitions, and barrel bombs;..." Tillerson was criticized for not bringing up Russia's alleged interference in the presidential election during his meeting with Lavrov.
Prisoners are held in a network of prisons across Syria. The complex is located just 45 minutes outside Syria's capital of Damascus, the seat of the Assad regime's power. A recent Amnesty International report described it as a "human slaughterhouse".
Trump's administration claims around 50 prisoners a day are killed at the military prison.
In February, Amnesty International said that mass hangings had taken place every week at the jail between 2011 and 2015. And in many cases, being killed themselves included also being tortured before being killed.
For years, United Nations human rights agencies have accused Assad of killing prisoners on a massive scale.