The Trump administration accused the Syrian government of carrying out mass killings of thousands of prisoners and burning the dead bodies in a large crematorium outside Damascus.
The Syrian government has been accused of carrying out mass killings of thousands of prisoners by the US government - which released satellite images to prove it.
The Trump administration claims the bodies of those killed were burned in a large crematorium attached to the Saydnaya prison complex outside the Syrian capital of Damascus.
The allegation matches an Amnesty International report released in February which claimed up to 13,000 people - mostly civilians opposed to the government - have been hanged in secret at the prison, according to Daily Mail.
Stuart Jones, the acting US assistant secretary for Near Eastern Affairs, said Syrian President Bashar Assad's government "has sunk to a new level of depravity" as he revealed the photographs.
The State Department says about 50 detainees a day are being hanged at the military prison, which is located about 45 minutes from Damascus.
"Credible sources have believed that many of the bodies have been disposed in mass graves," Jones told reporters as he showed them aerial photos of the alleged building.
"We now believe that the Syrian regime has installed a crematorium in the Sednaya prison complex which could dispose of detainees' remains with little evidence."
He added that the crematorium - construction on which began in 2013 -esd "an effort to cover up the extent of the mass murders taking place in Saydnaya."
One of the satellite images, provided by the State Department and taken from commercial satellites in January 2015, shows the building that the US says was modified to support a crematorium.
The snow on one section of the roof is melted in the photo, showing that it was taken in the winter.
Three other photos were also released, with two - taken in August 2013 and April 2016 - showing a side-by-side comparison of the building believed to house the crematorium, before and after it was altered.
The latter of those two images shows what are believed to be air intake vents, air conditioning units and a "discharge stack" that could be used to expel the smoke created by thousands of burning bodies.
Those photos do not definitely prove the building is a crematorium, but they show construction consistent with such use.
It's not clear why the US has waited until now to release this information, but Jones told reporters: "I would say that this information has been developing."
The final photograph, taken in April 2017, shows how close the alleged crematorium building is to the prison - easily just a few minutes' walk for a condemned man, and moments for a vehicle.
Jones cited US intelligence agencies and "credible" humanitarian organizations as sources for the claims about the crematorium.
He added that "The regime holds as many as 70 prisoners in Saydnaya in cells that have a five-person capacity."
It's not yet clear exactly where the prisoners are killed, but a judge who witnessed the executions told Amnesty International that they are taken in the night to another building.
There they are beaten before being hanged to death.
Some - described as "the young ones" - are too light for the rope, and so officers' assistants will pull down on their feet to break their necks.
Others are left hanging for 10-15 minutes.
Amnesty International released the shocking claims in February, along with statistics about the secret hangings, saying up to 13,000 people had been executed there.
It said the executions have happened at a rate of around 50 a day between 2011 and 2015 - the first five years of the country's ongoing civil war.
Assad's regime detained between 65,000 and 117,000 people over the same period, Jones said.
Lynn Maalouf, Deputy Director for Research at Amnesty International's regional office in Beirut, said: "The horrors depicted in this report reveal a hidden, monstrous campaign, authorized at the highest levels of the Syrian government, aimed at crushing any form of dissent within the Syrian population.
"We demand that the Syrian authorities immediately cease extrajudicial executions and torture and inhuman treatment at Saydnaya Prison and in all other government prisons across Syria.
"Russia and Iran, the government's closest allies, must press for an end to these murderous detention policies."
Jones echoed those thoughts, saying: "The United States is on record, has stated many times, that we are appalled by the atrocities that have been carried out by the Syrian regime.
"And these atrocities have been carried out seemingly with the unconditional support from Russia and Iran."
He clarified that Washington is not specifically accusing Moscow or Tehran of complicity in the alleged Saydnaya killings, but said Russia is aware of and has supported other abuses.
He also gave a warning to President Vladimir Putin's government: "Russia must now, with great urgency, exercise its influence over the Syrian regime to guarantee that horrific violations stop now."
He said he had not yet presented the evidence of the crematorium to Russian officials.
But he said that Secretary of State Rex Tillerson had raised concerns about Assad's brutality with Russia's foreign minister, Sergei Lavrovm during the latter's visit to Washington last week.
State department spokeswoman Heather Nauert said Tillerson had been "firm and clear" in a meeting with Russian foreign minister Sergey Lavrov last week that "Russia holds tremendous influence over Bashar al-Assad."
A main point of that meeting "was telling Russia to use its power to rein in the regime," she said.
Jones also said on Monday that the United States should not wholeheartedly believe a deal to set up "de-escalation zones" brokered by Russia during ceasefire talks in the Kazakh capital Astana last week.
The deal was reached with support from Iran and Turkey during ceasefire talks in the Kazakh capital of Astana earlier this month. Jones attended the talks.
"In light of the failures of the past ceasefire agreements, we have reason to be skeptical," he said.
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 America might take if Russia does not change its stance.
Tensions between the US and Russia heightened after President Donald Trump ordered a cruise missile strike in April against a Syrian air base that the US said had been used to launch a poison gas attack on civilians.
When Trump took up office he vowed to focus the US's Syrian efforts on defeating ISIS, which has used the chaos created by the civil war to seize land in the area.
He said at the time that he hoped to work with Putin, whose forces are in Syria to protect Assad's regime, to achieve that goal.
But relations with Moscow, already dire under Barack Obama, have not improved and Assad has continued to bombard civilian areas in his battle with opposition rebels.
And last month, when Assad was again accused of using the banned nerve agent sarin in a strike on his own people, Trump ordered a retaliatory US cruise missile strike on a Syrian air base.