The Islamic State militant group used a gorge in northeastern Syria to dump the bodies of people it had abducted or arrested, says Human Rights Watch.
Researchers started an investigation after a video was sent in 2014 showing militants throwing bodies into the deep al-Hota gorge of 50 m (164 ft).
HRW also claims that bodies have continued to be dumped there under IS law.
It needs to protect the site by local authorities, extract human remains and maintain evidence for prosecutions.
More than 20 mass graves containing thousands of bodies have been identified in areas of Syria previously controlled by IS. Activists, aid workers, journalists and civilians who run afoul of the jihadists are among those missing and presumed to have been killed.
IS once controlled 88,000 square kilometers (34,000 square miles) of land extending from western Syria to eastern Iraq, and imposed its brutal rule on nearly 8 million people.
A place of breathtaking natural beauty is the al-Hota gorge, which is about 85 km (53 miles) north of Raqqa’s former IS “capital.”
Yet as IS dominated the surrounding area from 2013 to 2015, it was “a site of terror and reckoning,” Sara Kayyali, researcher at HRW Syria said.