Regime forces were now within seven kilometres (less than five miles) of the IS-held airport at Tabqa, said the Syrian Observatory for Human Rights.
"There are fierce clashes now and at least nine pro-government fighters were killed," said the Observatory's Rami Abdel Rahman.
Abdel Rahman had no immediate toll for the number of IS jihadists killed.
Government fighters had been stuck at about 15 kilometres (10 miles) from the airport for more than one week.
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But intensifying Russian air strikes on Sunday and reinforcements from the elite Republican Guard unit allowed them to advance, the Observatory said.
Earlier this month, government fighters backed by Moscow began pushing north towards Tabqa, whose dam, military air base, and strategic transit town have been held by IS since 2014.
The town lies some 50 kilometres (31 miles) west of the jihadist group's de facto Syrian capital of Raqa city, and recapturing it would sever the supply road from the west.
IS fighters had mounted their own counter-attack last week, bringing in reinforcements of fighters and weapons and briefly blocking the regime advance before Sunday's renewed push.
The war in Syria -- which began with the brutal repression of anti-government demonstrations in 2011 -- has killed more than 280,000 people and displaced millions.
It has evolved into a brutal, complex civil war among the government, rebels, jihadists, and Kurds, each carving out zones of control.