The U.S. State Department has accused the Syrian government of President Bashar al-Assad of carrying out mass killings of thousands of prisoners and burning the dead bodies in a large crematorium outside the capital.
The accusations come a day before the start of a new round of UN-sponsored Syria peace talks in Geneva.
Stuart Jones, the top U.S. diplomat for the Middle East, said Washington's information came from credible humanitarian agencies and from the US "intelligence community" and it is thought that as many as 50 people per day are being hanged at Saydnaya military prison, about 45 minutes from Damascus.
The department released newly declassified photographs depicting what it says is a building in the prison complex that has been adapted to support the crematorium.
"We believe that the building of a crematorium is an effort to cover up the extent of mass murders taking place in Saydnaya prison," said Jones, who presented the photographs.
He did not give an official estimate for the total number killed, but cited an Amnesty International report from February, which said that between 5,000 and 11,000 had died between 2011 and 2015 in the prison.
Jones said on May 15 that Assad's government had "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 cease-fire and begin political talks.
"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," said Jones.
"And these atrocities have been carried out seemingly with the unconditional support from Russia and Iran."
Jones told reporters that U.S. Secretary of State Rex Tillerson had raised concerns about Assad's brutality with Russia's Foreign Minister Sergei Lavrov during his visit to Washington last week.
The declassified satellite pictures taken over several years, beginning in 2013, do not definitely prove the prison building is a crematorium, but they show construction consistent with such use.
As many as 70 prisoners are being held by the Syrian government in cells designed to hold just five, Jones said.
He also voiced reservations about a deal to set up four "de-escalation zones" inside Syria in an attempt to reduce violence and save lives. The deal was brokered by Russia with support from Iran and Turkey during talks in Kazakhstan's capital, Astana, last week.
"In light of the failures of the past cease-fire agreements, we have reason to be skeptical," Jones said.
"The [Assad) regime must stop all attacks on civilian and opposition forces. And Russia must bear responsibility to ensure regime compliance."
On May 15, ahead of the Geneva peace talks aimed at trying to find a solution to the Syria conflict which has killed more than 320,000 people since it broke out more than six years ago, UN mediator Staffan de Mistura said that the discussions in Switzerland should benefit from a tighter format -- and the recent Astana agreement to create the de-escalation zone.
However, observers say the UN appears to be scrambling to match Astana's momentum after the deal.
Since the deal came into effect a week ago, fighting appeared to have slowed in parts of the country.
Previous rounds of talks in Geneva have resulted in agreement that the warring sides will discuss a four-part agenda -- a new constitution, reformed governance, new elections and the fight against terrorism. However, there has been no progress yet on any topic, as the two sides have sharp disagreements over the meaning of the four points on the agenda.
Assad, backed by Russia and Iran, is keen on discussing "terrorism," a term he uses broadly to describe all adversaries.
Opposition representatives have demanded Assad's removal.
Last week Assad told the Belarusian TV channel ONT that the Geneva talks were "merely a meeting for the media" and there had been "nothing substantial in all the Geneva meetings."
With reporting by AP, Reuters, AFP, and dpa