The U.S. State Department said Monday that the Assad regime has installed a crematorium in Syria's Saydnaya prison, accusing the Syrian government of killing thousands of prisoners and burning the dead bodies.

>> Never again? U.S. allegations against Assad regime on prison crematorium spark Holocaust comparisons

The allegation came as President Donald Trump is weighing options in Syria, where the U.S. attacked a government air base last month in response to an alleged chemical weapons attack on civilians.

But Trump hasn’t outlined a larger strategy for ending the Arab country’s civil war or ushering Syrian President Bashar Assad out of power. These questions were sure to arise in his meeting at the White House with the United Arab Emirates’ crown prince Monday, a day before Turkey’s president arrives.

The State Department says about 50 detainees a day are being hanged at Saydnaya (Said-nay-ah) military prison, about 45 minutes from Damascus. It says the crematorium is being used to hide evidence of the extent of the killings.

Open gallery view A satellite image of what the U.S. State Department described as a building in a prison complex in Syria that was modified to support a crematorium. Credit: /AP

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The department is releasing newly declassified photographs showing what it says is a building in the prison complex that has been modified to support the crematorium.

In presenting the photographs, the top U.S. diplomat for the Middle East, Stu Jones, said Monday that Syrian President Bashar Assad’s government “has sunk to a new level of depravity” with the support of Russia and Iran.

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Jones also said the United States has "reason to be skeptical" about a deal to set up "de-escalation zones" brokered by Russia during ceasefire talks in the Kazakh capital Astana last week.

Open gallery view A satellite image of what the U.S. State Department described as a building in a prison complex in Syria that was modified to support a crematorium. Credit: /AP

In a report covering the period from 2011 to 2015, Amnesty International claims Syrian authorities have killed as many as 13,000 people in mass hangings in the prison known to detainees as "the slaughterhouse."

Jones said Assad's government had carried out air strikes, chemical attacks, extrajudicial killings, starvation, and other measures to target civilians and its opponents. He criticized Russia and Iran for maintaining their support for Assad despite those tactics.

"These atrocities have been carried out seemingly with the unconditional support from Russia and Iran," Jones said. "The (Assad) regime must stop all attacks on civilian and opposition forces. And Russia must bear responsibility to ensure regime compliance."

He did not say what measures the United States might take if Russia does not change its stance.

Tensions between the United States and Russia heightened after President Donald Trump ordered a cruise missile strike in April against a Syrian air base that the United States said had been used to launch a poison gas attack on civilians.

Jones said he had not yet presented the evidence to Russian officials. He said he hoped Russia would help pressure the Assad government.

White House Press Secretary Sean Spicer said on Monday during his daily press briefing that the Trump administration is willing "to work with Russia and Iran" in order to find a solution to the situation in Syria, if those two countries would "acknowledge the atrocities of the Assad regime and use their influence to stop it." Spicer added that "Syria will never be stable and secure as long as Assad is in power," but refused to answer directly if the Trump administration was now officially committed to regime change in Syria.