US accuses Syria of sinking to 'new level of depravity' with mass killings
The Trump administration has accused the Syrian government of carrying out mass killings of thousands of prisoners and burning the bodies in a large crematorium outside the capital Damascus.
It also stepped up criticism of Iran and Russia for supporting the Syrian government.
The allegation came as President Donald Trump weighs options in Syria, where the US attacked a government air base last month in response to an alleged chemical weapons attack on civilians.
But Mr Trump has not outlined a larger strategy for ending the Arab country's civil war or ushering Syrian president Bashar Assad out of power.
The state department said it believed about 50 detainees a day are being hanged at Saydnaya military prison, about 45 minutes north of Damascus.
Many of the bodies, it said, are then burned in the crematorium.
"We believe that the building of a crematorium is an effort to cover up the extent of mass murders taking place," said Stuart Jones, the top US diplomat for the Middle East.
He accused the Syrian government of sinking "to a new level of depravity".
The department released commercial satellite photographs showing what it described as a building in the prison complex that was 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.
In presenting the photographs, Mr Jones called on Russia and Iran to press Assad's government to establish a credible ceasefire with Syrian rebel groups and begin negotiations on a political settlement.
"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," Mr Jones said.
State department spokeswoman Heather Nauert said secretary of state Rex 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.
"Simply put, the killing, the devastation has gone on for far too long in Syria."
The war has killed as many as 400,000 people since 2011.
It has contributed to Europe's worst refugee crisis since the Second World War and enabled Islamic State to emerge as a global terrorism threat.
Mr Trump travels to the Middle East later this week on his first official foreign trip.