The Syrian government has rejected a report which claims to show the systematic torture and killing of around 11,000 prisoners, calling it "politicised" and an attempt to undermine peace talks in Geneva.
The 31-page dossier was released on Monday and contains 55,000 pictures of corpses bearing signs of torture and strangulation.
The former war crimes prosecutors who authored the report said Syrian officials could face war crimes charges as a result of the evidence gathered.
However, Syria's Ministry of Justice said the report was "lacking objectivity and professionalism".
A ministry statement on Syrian state television said: "Releasing the report one day before the 'Geneva 2' conference provides categorical evidence that its goal...is to undermine efforts aimed at achieving peace in Syria."
Syrian opposition leader Ahmad Jarba claimed pictures that are said to document the execution and torture of prisoners by the government are similar to crimes committed by the Nazis in World War Two.
ITV News Middle East correspondent Geraint Vincent tweeted:
Syria Info Minister tells me torture photos manufactured outside Syria. Has a team working on proving it #Geneva2
The mother of a British doctor who died in Syria has said she is "not surprised" by the killing and torture allegedly documented in some 55,000 images smuggled out of the country.
Fatima Khan told ITV News that the priority must be to save the prisoners still being held by the Syrian government to ensure that no more people suffer her son's fate.
A spokeswoman for the US State Department has described recent images that appear to document executions and torture in Syria as "extremely disturbing".
"They illustrate apparent actions that would be serious international crimes," spokeswoman Marie Harf said, adding that she sees no reason to doubt the veracity of the prosecutors' report on the images.
"It certainly is in line with everything we know about the Assad regime," Harf told a news briefing.
A Syrian government source has said that a cache of 55,000 images, apparently documenting torture and executions by the Assad regime, is an attempt by Qatar to attack the Syrian government:
There is no proof about these photos. There is no proof these photos were even taken in Syria ... The report is from the government of Qatar who are not neutral ... They send money and weapons to the terrorists who are fighting in this country ... Look at the timing, this is designed to damage the talks, to attack the government of Syria and undermine the peace talks.
The images of the "systematic torture and killing" of prisoners by Syria's Assad regime are "the tip of the iceberg" according to leading lawyer Sir Desmond de Silva QC.
Sir Desmond said that the pictures represented a small number of people in one area and that the abuse was likely more widespread.
William Hague called said that those responsible for the "horrific" images should be held to account.
The Foreign Secretary said:
"We have done a great deal... this (publication) has been done with the assistance of the Qatari government so not entirely a private initiative.
"I've seen a lot of this evidence, it is compelling and horrific and it is important those who have perpetrated these crimes are one day held to account.
"The United Kingdom has done a great deal in the documentation of human rights abuses. Part of the support we have given to moderate political forces in Syria is to train human rights activists in the recording and documentation of crimes, many of which have therefore come to the world's attention.
"We will do more of that."