Officials say up to 50 prison detainees are being killed every day
The United States has accused the Syrian government of murdering thousands of prisoners and burning their bodies in a large crematorium.
Officials claim as many as 50 detainees are being hanged at Saydnaya military prison outside Damascus every day, and the crematorium was built to "dispose of detainees' remains with little evidence".
Stuart Jones, from the US State Department, said: "Although the regime's many atrocities are well documented, we believe that the building of a crematorium is an effort to cover up the extent of mass murders taking place in Saydnaya prison."
The department has released satellite photographs of the military prison - and an image from January 2015 shows how snow had melted away from just one section of the building.
Although the pictures do not prove that the building is a crematorium, it does show construction consistent with such use.
Mr Jones said the US is planning to present the evidence to the international community, and he warned Syrian President Bashar al-Assad's government "has sunk to a new level of depravity".
Although he did not offer an official estimate for the total number of detainees killed at Saydnaya, an Amnesty International report has suggested that as many as 13,000 inmates died between 2011 and 2015.
In February, the human rights group also claimed systematic torture had been carried out at the military jail, which is referred to by detainees as "the slaughterhouse".
Some inmates were allegedly executed after a "sham trial" lasting no more than a couple of minutes. The killings were often authorised by senior Syrian officials, including Assad's deputies.
Amnesty's report described the executions as a war crime and called for a UN investigation.
It said most of the victims had been civilians who opposed the government, and the mass killings had been designed to "crush any form of dissent within the Syrian population".