The jihadist group Islamic State used a gorge in north-eastern Syria to dump the bodies of people it had abducted or detained, Human Rights Watch says.
Researchers began an investigation after being sent a video in 2014 showing militants throwing corpses into the 50m (164ft) deep al-Hota gorge.
HRW also believes bodies continued to be dumped there following IS rule.
It wants local authorities to secure the site, remove human remains, and preserve evidence for prosecutions.
More than 20 mass graves containing thousands of bodies have been found in areas of Syria formerly held by IS.
Among those missing and believed to have been killed are activists, humanitarian workers, journalists, and residents who ran afoul of the jihadists.
IS once controlled 88,000 sq km (34,000 sq miles) of land stretching from western Syria to eastern Iraq, and imposed its brutal rule on almost eight million people.