The Donald Trump administration on May 15 accused Bashar al-Assad's regime of building a crematorium at the Saydnaya prison complex outside Damascus in an effort to destroy evidence of mass murder. In pointed comments just days after Russian Foreign Minister Sergey Lavrov met with the president at the White House, US officials said Moscow and Tehran must rein him in before the United States can support their diplomatic efforts in Syria.

“The Assad regime has sunk to a new level of depravity, and it has done so with seemingly unconditional support from Russia and Iran,” White House Press Secretary Sean Spicer told journalists at his White House press briefing.

In a shift, the Trump White House suggested it was willing to work not only with Russia but also with Iran to try to end the killing in Syria and advance a political transition if they helped bring an end to Assad’s atrocities. Trump has previously lambasted the nuclear deal with Iran and supported calls for additional sanctions over the country's ballistic missile program.

“The United States remains open to working together with both Russia and Iran to find a solution that leads to a stable and united Syria,” Spicer said. “But in order for us to work together to bring an end to the violence in Syria, Russia and Iran need to acknowledge the atrocities of the Assad regime and use their influence to stop them.”