The Syrian Observatory for Human Rights said 346 jihadists and 25 Syrian troops had been killed in the fighting at the airport since last Tuesday, with another 170 soldiers dying during fierce clashes today.
"One hundred and seventy Syrian soldiers were killed today in the offensive which led to the IS jihadists seizing Tabqa airport," the Observatory said in an email to AFP.
Syrian state television said troops had staged an "evacuation" of the airport.
The airport's capture came after IS fighters launched a fourth assault on Tabqa overnight, in a bid to cement their control over Raqa province.
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Earlier, Observatory director Rami Abdel Rahman said the jihadists had begun advancing into the airport, hanging up the head of a decapitated soldier at one of the checkpoints they had seized.
Islamic State fighters have been trying to take Tabqa since early August, but began the airport assault in earnest on Tuesday.
Raqa has become the stronghold of the Islamic State, which controls the provincial capital and has declared an Islamic "caliphate" in territory it holds in Syria and Iraq.
The group initially fought alongside Syrian opposition groups, but its abuses sparked a backlash from rebels who pushed it out of parts of northern Syria.
In recent weeks, though, IS has advanced back into areas it withdrew from, including in northern Aleppo province.
The monitoring group said there was no official reason given for the withdrawal, but that the militants appeared to be moving to areas under tighter IS control, including Deir Ezzor province in the east.