The Britain-based Syrian Observatory for Human Rights, which relies on a network of sources on the ground, said the toll comprised 123 soldiers and loyalist militiamen, 115 IS fighters and 57 civilians.
It said dozens of the civilians had been executed.
The toll came as Syrian government forces Sunday pushed IS jihadists back from Palmyra, a UNESCO world heritage site, a day after the fighters had seized the northern part of the modern town.
Fighting was still ongoing Sunday in the northern suburb of Palmyra and around Haql al-Hail gas field northeast of the city, the Observatory and IS said.
IS seized Haql al-Hail on Saturday, the Observatory said, while the jihadist group posted pictures online of what it said was one of two checkpoints inside the gas field that it had overrun.