ISIS killed 128 people it suspected of collaborating with the Syrian regime in al-Qaryatain this month before losing the desert down to government forces, a monitor said Monday.

"ISIS has over a period of 20 days executed more than 116 civilians in reprisal killings, accusing them of collaboration with regime forces," said Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights monitor.

The Observatory later put the death toll at 128.