At least 13 of the victims were children, said the Syrian Observatory for Human Rights, a Syrian expat watch group.
As ISIS has taken control of the Syrian city of Palmyra in the last 10 days, it has executed 217 people there and in other towns caught in the military campaign, SOHR said.
Most of the victims, 150, were Syrian soldiers or allied militiamen -- or others who were accused of acting as their informers. Beheading was the preferred method of their execution.
But 67 of those killed were residents, including those children, SOHR said. Their crime? "Dealing with the regime forces, and hiding regime's members in their homes."
ISIS also executed five nurses in Palmyra, SOHR said. And they are holding some 600 detainees, whom they have accused of similar things.
Elsewhere, in the Deir Ezzor province, 45 more people were executed in the last four days. The accusations against the victims were similar, and some were related to the Palmyra campaign.
But some of the execution victims in Deir Ezzor were accused of apostasy.
The city of Palmyra itself could become one of ISIS' next casualties.
ISIS despises antiquity, since it represents culture that predates Islam, and extremist militants have vandalized some of the most ancient artifacts of humanity in Iraq and Syria. The terrorist group has flattened the ancient Assyrian city of Nimrud and smashed statues in Iraq's Mosul Museum.
Palmyra is a UNESCO World Heritage Site and was an ancient cultural crossroad city founded over 2,000 years ago. Its art and architecture unites Greek, Roman and Persian influences, UNESCO says.
Extensive destruction of Palmyra wouldn't just be a tragedy for Syria. It would be a loss for the world, said novelist and historian Tom Holland.
"Mesopotamia, Iraq, Syria, this is the wellspring of global civilization," he said. "It really couldn't be higher stakes in terms of conservation."
When ISIS took the town and neighboring villages, some 11,000 people fled. That would account for virtually all of the city's population.