The Syrian government has constructed and is using a crematorium inside its notorious Sednaya military prison near Damascus to clandestinely dispose of the bodies of prisoners it continues to execute inside the facility, the State Department said Monday.

Thousands of executed detainees have been dumped in mass graves in recent years, acting Assistant Secretary of State Stuart Jones said. “What we’re assessing is that if you have that level of production of mass murder, then using the crematorium would ... allow the regime to manage that number of corpses ... without evidence.”

“We believe that the building of a crematorium is an effort to cover up the extent of mass murders taking place in Sednaya prison,” he said.

The Syrian government, Jones said, “has treated opposition forces and unarmed civilians as one and the same,” continuing to “systematically abduct and torture civilian detainees, often beating, electrocuting and raping these victims,” and authorizing “the extrajudicial killings of thousands.”

The State Department distributed overhead photographs it said documented the gradual construction of the facility outside the main prison complex and its apparent use this year. Jones said that “newly declassified” information on this and other atrocities by the government of President Bashar Assad came from “intelligence community assessments,” as well as from non-governmental organizations such as Amnesty International and the media.

“These atrocities have been carried out seemingly with the unconditional support from Russia and Iran,” Assad’s main backers, Jones said. Neither government commented on the new U.S. allegation.

Charges of mass murder and incinerated bodies, evoking the Holocaust, contrasted with last week’s Washington visit by Russian Foreign Minister Sergey Lavrov and Russian Ambassador Sergey Kislyak. They were pictured shaking hands and broadly smiling with President Donald Trump before an Oval Office meeting in which discussions centered on Syria.

The Russians also met with Secretary of State Rex Tillerson, and Jones said the release of new intelligence comes at “an opportune time to remind people about the atrocities that are being carried out inside of Syria all the time.”

The newly released information included a satellite photo of the snow-covered Sednaya complex with an L-shaped building labeled “probable crematorium.” Assessment of the facility, he said, included the presence of “the discharge stack, the probable firewall, the probable air intake — this is in the construction phase — this would be consistent if they were building a crematorium.” In a photo taken Jan. 15, he said, “we’re look(ing) at snowmelt on the roof that would be consistent with a crematorium.”

Jones said the information had not been shared with the Russians. He also said he was not suggesting that either Russia or Iran was involved with the facility. But Tillerson, he said, “was firm and clear with Minister Lavrov. Russia holds tremendous influence over Assad. A key point that took place in that bilateral meeting was telling Russia to use its power to rein in the regime.”

Jones called Tillerson’s meeting with Lavrov “productive.” But “I would not say that they mapped out a specific way forward on how to address the issue of Syrian atrocities, or even how to move forward on the Geneva process” on the eve of the next round of years’ long United Nations efforts to bring representatives of Assad and the rebels to the negotiating table due to begin Tuesday.

One of Lavrov’s principal goals in last week’s meetings was to solicit Trump administration support for a cease-fire and the establishment of safe zones within Syria as part of a May 4 pact signed by Russia, Iran and Turkey. The Turkish government has backed anti-Assad rebels in Syria along with the United States, and Turkish President Recep Tayyip Erdogan is due to meet with Trump at the White House on Tuesday.