WASHINGTON (AP) — 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.

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.

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.