Islamic State jihadists on Sunday won a bloody battle for Tabqa military airport, last stronghold of the Damascus regime in Syria's northern province of Raqa, a monitoring group and state media said.
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 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.
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.
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.
The airport was the last army stronghold in Raqa, after jihadists captured Brigade 93 and Division 17 in the northern province, killing dozens of soldiers, many of whom were beheaded.
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.
Elsewhere, however, the Observatory said IS fighters were pulling out of parts of central Homs 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.
The Observatory said several areas in Homs that had been under IS control were taken over in the wake of the withdrawal by rival jihadist group Al-Nusra Front, which is Al-Qaeda's Syrian affiliate.
Elsewhere, both the Observatory and Syrian state media reported an army ambush on a rebel convoy in the southern province of Daraa.
The Observatory said at least 32 rebels were killed in the attack, with 26 others wounded and 12 missing.
State television said "tens of terrorists" died in the ambush in the west of the province, between the towns of Hara and Zamreen.
Hara is under the control of regime forces, but Zamreen is under rebel control like much of the surrounding area, although there have been recurring clashes there in recent weeks.