More than 500 Syrian soldiers and Islamic State fighters died in a six-day battle for Tabqa airport, last stronghold of the Damascus regime in the northern province of Raqa, a monitor said Sunday.
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 fighting on Sunday.
"One hundred and seventy Syrian soldiers were killed on Sunday in the offensive which led to the IS jihadists seizing Tabqa airport," the Observatory said in an email to AFP.
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Syrian state television said troops had staged an "evacuation" of the airport.
"After heavy fighting by the forces defending the Tabqa airport, our forces implemented a regrouping operation after the evacuation of the airport," the broadcaster said in a breaking news alert.
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.
The airport was the last military position under army control in Raqa, after jihadists captured Brigade 93 and Division 17, killing dozens of soldiers, many of whom were beheaded.