A woman and a member of the Syrian government forces were among the victims, said the Syrian Observatory for Human Rights.
The 75 workers were kidnapped by the IS near Palmyra city on Saturday while they were collecting truffles, according to the observatory.
The fate of 64 others kidnappees remains unknown, the Britain-based watchdog noted.
The observatory said the IS took advantage of the world's preoccupation with the devastating earthquakes that struck Türkiye and Syria and killed the victims.
