In an exclusive interview with CNN's Christiane Amanpour, the defector, code-named "Caesar," urged US President Donald Trump to stop what he calls the "criminality" taking place in Syria's government-run prisons.
"We have shown the killing and torture of so many of the Syrian people," he said, "and you cannot give back the lives to those that have lost it. But we ask you, out of your humanity, to stop the machinery of death."
"We are asking to all the officials, to all the policy makers, to President Trump's White House, which we are hoping will do the right thing, we beg you to stop the machinery of death in Syria."
Caesar worked as a photographer for the Syrian military police in Damascus. CNN first broke the news of his defection and the trove of photos he smuggled out in a joint exclusive with the Guardian in 2014.
At the time, a team of internationally renowned war crimes prosecutors and forensic experts who analyzed the photos found evidence of "systematic torture and killing" by Syrian President Bashar al-Assad's regime.
Human Rights Watch has since investigated the photos and said they "are confident the Caesar photographs present authentic -- and damning -- evidence of crimes against humanity in Syria."
There are now criminal cases based on Caesar's evidence filed in Spain, Germany, and France, according to Stephen Rapp, former US Ambassador-at-Large for War Crimes.
Caesar tried to get the Obama administration's attention when he testified before Congress in 2014. He's now in Washington again for meetings with members of Congress -- and at the White House -- as the Syrian war enters its seventh year and as the Trump administration conducts its Syria policy review.
"I was horrified, I was terrified every day of the job that I was doing," he said. "I would look at the different horrendous ways that these individuals were slaughtered and tortured to death."
"We saw a lot of things that were made so (that) people would die very slowly under torture or being starved for long periods of time, and these bodies and victims were coming from all of these different intelligence branches," he added.
Caesar says he found himself taking photographs of up to 50 bodies a day, and recounted how he would often picture himself as one of the dead.
But he said he could not "stand by and let this happen" and ended up copying the photos onto flash drives, which he managed to smuggle out of the country.
The Syrian regime has consistently denied allegations of human rights abuses and war crimes.
In an interview with Foreign Affairs magazine in 2015, President Assad called into question the authenticity of the photos, saying: "You can bring photos from anyone and say this is torture. Who took the pictures? Who is he? Nobody knows. There is no verification of any of this evidence, so it's all allegations without evidence."
Rapp recently told Amanpour that the evidence against the Assad regime "is massive and overwhelming far better than they had, frankly, at Nuremberg or in The Hague at the Yugoslavia tribunal or that we had in Africa and in Sierra Leone with Charles Taylor or in Arusha with the genocide trials against the Rwandan leaders."
"This is very strong evidence that would make for a great trial, certainly, if we could organize them at the international level," Rapp added.