Islamic State fighters have executed more than 160 Syrian soldiers it captured during its storming of a key northern air base this week, a monitoring group said on Thursday.
The jihadists boasted on Twitter that they had killed 200 defeated troops and posted video of what they said was the garrison in headlong flight.
"IS executed more than 160 Syrian soldiers in three different places in Raqa province yesterday and at dawn today," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.
The soldiers were fleeing the jihadists' seizure of the Tabqa base on Sunday, which cemented IS control over Raqa province, capital of its self-declared Islamic "caliphate".
Abdel Rahman said the defeated garrison comprised 1,400 soldiers, 200 of whom were killed and 700 of whom managed to escape.
Of the other 500, dozens were captured on Wednesday night as they attempted to cross the desert to government-held territory in the Orontes Valley to the west.
IS posted video footage showing young men in underwear being marched barefoot along a desert road. Militants shouted "Islamic State" and "There's no going back".
In Syria, the group controls all of Raqa province and much of Deir Ezzor further down the Euphrates Valley towards the Iraqi border.
It also controls most of the Sunni Arab heartland of neighbouring Iraq, north and west of Baghdad, including second city Mosul.
It has repeatedly posted often gruesome videos, both as a warning to those joining up to the Syrian army and as a propaganda tool to recruit volunteers from the wider Islamic world.