(The Washington Post)

The Syrian government has constructed and is using a crematorium at 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, said acting assistant secretary of state Stuart Jones. “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 in a briefing for reporters.

The Syrian regime, 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 satellite 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 al-Assad came from “intelligence community assessments,” as well as from nongovernmental 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.

Accusations of mass murder and incinerated bodies, evoking the Holocaust, contrasted with last week’s Washington visit by Russian Foreign Minister Sergei Lavrov and Russian Ambassador Sergey Kislyak. They were pictured shaking hands and broadly smiling with President 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, Jones 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 Bashar al-Assad. A key point that took place in that bilateral meeting was telling Russia to use its power to rein in the regime.”

“The regime must stop all attacks on civilians and opposition forces, and Russia must bear responsibility to ensure regime compliance,” Jones said.

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.

Although Trump has also called for safe zones within Syria and said he discussed them early this month in a telephone call with Russian President Vladi­mir Putin, the administration more recently has been publicly lukewarm about the Russia-led plan.

“In light of the failures of the past cease-fire agreements, we have reason to be skeptical,” Jones said. Earlier truces negotiated under the Obama administration were violated by both Syria and Russia.

Throughout his presidential campaign, Trump said the United States should concentrate on the fight against the Islamic State in Syria and Iraq and cease involvement in Syria’s civil war. But he changed course last month, approving a cruise missile strike on a Syrian government air base after concluding it was used by Assad to launch a chemical weapons attack against civilians.

Jones described “the continued brutality of Assad” as a threat to the region, “as well as to the national security interests of the United States and our allies.” Asked if there is consideration of military action to destroy the crematorium, he said, “We’re not going to signal what we are going to do and what we’re not going to do.”

“At this point, we are talking about this evidence and bringing it forward to the international community, which we hope will put pressure on the regime to change its behavior,” Jones said.

Cease-fires under the Russia-Iran-Turkey agreement, in designated parts of northwestern, central and southern Syria, have largely held in recent weeks. But violence continues on other fronts not included in the plan, and suggests that Assad’s forces are positioning themselves to launch an all-out assault on the largest of the safe zones, Idlib province, when the deal breaks down. If that happens, almost a million displaced civilians could find themselves caught in the crossfire between pro-government forces and an al-Qaeda-linked coalition that appears willing to fight until the end.

More than 400,000 people have died in the Syrian civil war, according to the United Nations, with at least half the entire prewar population of about 22 million now living as refugees or displaced from their homes. Many of the dead are civilians killed by government action, including, Jones said, “well-documented airstrikes and artillery strikes, chemical weapons attacks, arbitrary arrests, extrajudicial killings, starvation, sexual violence, and denial of essential services such as food, water and medical care.”

According to “numerous” ­nongovernmental organizations, Jones said, “the regime has abducted and detained between 65,000 and 117,000 people between 2011 and 2015,” a period in which Amnesty International has said that nearly 18,000 detainees died. The Syrian Network for Human Rights estimated in March that at least 106,727 people were still arrested or had been forcibly disappeared.

Prisoners are held in a network of prisons across Syria. The Sednaya detention complex, run by Syria’s powerful military police about 20 miles outside Damascus, is the most notorious. A recent Amnesty International report described it as a “human slaughterhouse.”

Jones cited “multiple sources” in saying that “the regime is responsible for killing as many as 50 detainees per day at Sednaya,” where he said up to 70 people were packed in cells designed for five. Former prisoners have described mass hangings.

In interviews with The Washington Post, former detainees described conditions so atrocious that many prisoners died from torture, medical neglect or starvation.

Most political prisoners said they had been held in the “Red Building,” a facility the regime largely emptied of mostly Islamist and jihadist prisoners in the early months of the anti-Assad uprising that began in early 2011. Among those taken from the cells and hanged, former prisoners said, were students, engineers, activists and human rights lawyers.

Louisa Loveluck in Beirut contributed to this report.