"Dozens of Syrian soldiers captured while fleeing... after the IS overran Tabqa airbase were executed by the jihadists during the night," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.
The jihadists seized the airport on Sunday after weeks of bitter fighting with loyalist forces, cementing their control over Raqa province, capital of their 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.
The other 500 remain on the run. 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.