The Syrian Observatory for Human Rights said IS had launched a major push late Tuesday on Tabqa military airport, the last remaining army stronghold in Raqa, which has been largely overrun by the jihadists.
The assailants had carried out car bomb attacks in their assault but failed to make any breakthrough, the Britain-based group said.
"At least 70 IS fighters have been lost since early Wednesday in regime air raids, Scud missile blasts and mine explosions," Observatory head Rami Abdel Rahman told AFP.
He said the military was also using barrel bombs to attack IS from the air, the same improvised and crude weaponry it has deployed against rebel-held areas of the northern city of Aleppo with deadly impact.
On Thursday, the army airlifted reinforcements into Tabqa by helicopter, said Abdel Rahman, whose group relies on information from activists and medics on the ground.
Rebel groups have in the past accused the Damascus regime of avoiding attacks on IS as the Sunni extremist group battles more moderate anti-regime forces.
IS initially worked alongside other rebel groups in Syria, but they turned against it because of its abuses and its bid to dominate seized territory.
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