At least 24 jihadists have been killed and 150 wounded in clashes with Syrian troops defending an air base in Raqa province, a monitoring group said on Saturday.
The clashes came as the Islamic State group launched a new bid to capture the Tabqa base, the government's last bastion in the northern province, the rest of which is in the hands of the jihadists.
The Syrian Observatory for Human Rights said the attack began overnight when a suicide bomber blew himself up at the entrance to the base, prompting clashes that killed 14 IS fighters.
The two sides exchanged rocket, artillery and heavy machinegun fire, and the army deployed air strikes against the attacking jihadists, the Britain-based group said.
The air raids continued into the morning, with eight strikes against the town of Tabqa that killed 13 people, among them an additional 10 IS fighters.
Those deaths brought IS losses since it began its assault on Tabqa to at least 94, with more than 400 wounded, according to the Observatory.
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IS has brought in reinforcements from other areas it controls in Syria, as well as neighbouring Iraq, the Observatory added.
The assault on Tabqa comes after IS seized the army's Brigade 93 and Division 17 posts in Raqa, killing dozens of soldiers, some of whom it beheaded.
The army has airlifted reinforcements to the base and stepped up air strikes against IS positions across Raqa, using both precision rockets and barrel bombs.
On Saturday afternoon, Syrian state television broadcast images from Tabqa showing soldiers it said were "ready to repel any new attack".
It also showed the bodies of what it said were "terrorist mercenaries of the Islamic State".
"These terrorists have tried to attack the airport several times in recent days but have been repelled by the force and bravery of the army heroes," it added.
"The Tabqa military airport is secure, the army is defending it and launching attacks against the armed men outside the base," an officer interviewed at the base told the state broadcaster.