ISIS killers used "sharp tools" to murder 33 people in Syria on Wednesday before dumping the bodies into a mass grave "filled with blood," a human rights group said.

The Syrian Observatory for Human Rights said the slaughter was "the largest execution operation carried out by [ISIS] in 2017."

The identities of the dead -- men ages 18-25 -- were not immediately known, and it was unclear if they were civilians or captured soldiers. "Marks of slaughter" appeared on the victims’ necks, the activists said.

It was just the latest horror to unfold in Syria this week. International outrage is growing over the harrowing gas attack on the town of Khan Sheikhoun in northern Syria, which killed at least 86 people.

The ISIS killings unfolded near the town of Mayadeen in Deir el-Zour province, close to the Syrian border with Iraq. Opposition activist Omar Abu Laila, who is from Deir el-Zour but currently lives in Europe, also reported the murders.

ISIS has carried out similar killings in Syria this year. When it controlled the ancient Syrian city of Palmyra, ISIS routinely carried out public beheadings, including the killing of the antiquities chief, whose body was hanged from a pole in a main square.