Monitors say Syrian government planes carried out at least 15 strikes in and around the central city of Palmyra early on May 25, targeting buildings captured by the Islamic State (IS) militant group.
IS fighters overran the ancient city, the site of some of the world's best preserved Roman ruins, last week.
The British-based Syrian Observatory for Human Rights said they have killed at least 217 people execution-style in the area since May 16, including children.
The observatory says the IS had detained around 600 soldiers, pro-government fighters, and those accused of being loyalists in and around the city, which stands on a crossroads to the cities of Damascus and Homs.
Observatory founder Rami Abdulrahman said the Syrian Air force carried out raids on May 25 on targets, including the military intelligence building and the city hospital.
He said there were reports of casualties, but he had no firm toll.