An Islamic State group counter-attack on Syrian regime forces advancing towards a jihadist stronghold on Monday left 28 combatants dead, a monitor said.
Dozens of IS fighters early on Monday attacked government positions south of the town of Tabqa, an IS stronghold on the Euphrates River, said the Syrian Observatory for Human Rights.
Observatory head Rami Abdel Rahman said fierce clashes broke out, but that regime forces were able to maintain their positions and were still 15 kilometers (10 miles) from the IS-held Tabqa airport.
"Eleven regime fighters and 17 Daesh fighters were killed in the attack," said Abdel Rahman, using the Arabic acronym for IS.
Earlier this month, government fighters backed by Russian air power began pushing north towards Tabqa, held by IS since 2014.
The town lies some 50 kilometers (31 miles) east of the jihadist group's Syrian de facto capital of Raqa city, and recapturing it would sever the supply road from the west.
Last week, IS fighters had sent reinforcements of fighters and weapons into Tabqa in anticipation of a fierce battle to defend the town.