Islamic State fighters and Syrian regime troops have clashed in fierce battles that reportedly cost hundreds of lives over the Tabqa military air base, the last stronghold of the Syrian army in Raqqa province.
The Syrian Observatory for Human Rights reported on Sunday afternoon that fighters were in control of the base but that clashes were still taking place.
Syrian state media later reported that its forces, backed by warplanes, were attacking the rebels after leaving the area and regrouping.
State television said army troops were launching "precision strikes" against "terrorist groups in the area, inflicting heavy losses".
Regime forces had repelled three previous attacks on the base in the previous week.
The air base is one of the most significant government military facilities in the area, containing several warplane squadrons, helicopters, tanks, artillery and ammunition.
The Observatory said at least 346 Islamic State fighters were killed and more than 170 members of government forces had died since Tuesday in the fight over Tabqa base, making it one of the deadliest confrontations between the two groups since the start of Syria's war in 2011.
Rami Abdel Rahman, director of the Britain-based Observatory, said the fighters had also seized several checkpoints, hanging up at one the head of a soldier who had been killed in the fighting and decapitated.
The battle for Tabqa began earlier this month, with the Islamic State launching its first major assault against the airport on Tuesday. It is the last army stronghold in the Raqqa, after fighters captured Brigade 93 and Division 17 in the northern province, killing dozens of soldiers, many of whom were beheaded.
Raqqa province 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, the group has advanced back into areas it withdrew from, including northern Aleppo province. Elsewhere, however, the Observatory said Islamic State fighters were withdrawing from parts of central Homs province on Sunday.
The monitoring group said there was no official reason given for the withdrawal, but that the fighters appeared to be moving to areas under tighter Islamic State control, including Deir Ezzor province in the east.