WASHINGTON, D.C. --The United States is attempting to ratchet up pressure on Russia to push the Syrian government into peace talks with rebels.
On Monday, the U.S. accused Syria of executing thousands of imprisoned political opponents and burning their bodies in a crematorium to hide the evidence.
The U.S. State Department revealed photos of a crematorium built near a prison outside Demascus.
But the decision to release newly declassified information supporting the allegation may also test the Trump administration's own willingness to respond to atrocities in Syria, other than chemical weapons attacks, which it blames on President Bashar Assad's government.
Syria's government says it "categorically" denies the accusations of mass killings at the prison.
The Foreign Ministry in Damascus described the U.S. State Department accusations as "a new Hollywood plot" and "lies" that are being used to justify U.S. "aggression and intervention."
The statement was released on Tuesday.
Western monitors and watchdog groups said they have accumulated evidence of mass killings in Syrian government prisons. However, there hasn't been any substantiated allegation so far of the use of a crematorium.
The accusation of mass killings and efforts to cover them up came as President Donald Trump weighs options in Syria, where the U.S. launched cruise missiles on a government air base last month after accusing Assad's military of killing scores of civilians with a sarin-like nerve agent.
The Associated Press and CNN contributed to this report.
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