The Syrian regime is using a site outside Damascus to cremate the bodies of thousands of prisoners it has abducted, jailed and murdered during the country’s long-running civil war, the U.S. State Department alleged Monday.

Acting Assistant Secretary for Near Eastern Affairs Stuart Jones showed surveillance photos that -- combined with intelligence assessments and other reports -- officials believe show Bashar Assad’s government is complicit in covering up evidence of mass killings at the Sednaya Prison. Located near Damascus, the prison previously has been called a "human slaughterhouse" by Amnesty International.

“Although the regime's many atrocities are well-documented, we believe that the building of a crematorium is an effort to cover up the extent of mass murders taking place in Sednaya prison,” Jones said Monday.

REPORT ACCUSES SYRIAN REGIME OF CARRYING OUT MASS HANGINGS

Jones also charged that the “atrocities” were carried out “with unconditional support” from Russia and Iran.

The department released commercial satellite photographs showing what it said is a building in the prison complex that has been modified to support the crematorium. The photographs taken over the course of several years, beginning in 2013, do not definitely prove the building is a crematorium, but they show construction consistent with such use.

The U.S. conclusion is based in part on photos that show snow melting near the building believed to be the crematorium. But, when pressed, the State Department wouldn’t say for certain that the melting was caused by a crematorium.

The conditions at Sednaya – known to hold many of Assad’s political opponents, as well as violent Syrian rebels – are thought to be horrific, with 70 prisoners kept in cells designed for five people and up to 50 executions per day.

Amnesty International has previously called Sednaya a “human slaughterhouse,” estimating 13,000 people were killed there from 2011-2015. It is believed Assad's regime kidnapped more than 100,000 people during that time.

In presenting the photographs, Jones said Syrian President Assad's government "has sunk to a new level of depravity" with the support of Russia and Iran and called on both countries to use its influence with Syria to establish a credible ceasefire and begin political talks.

Fox News’ Rich Edson and The Associated Press contributed to this report.