The Syrian army has repelled an attack by the Islamic State group on one of its key air bases, as Damascus attempted to fend off an offensive against its last bastion in Raqqa province.

Syrian troops defending the Tabqa air base killed at least 24 fighters and wounded 150 others during clashes on Saturday, the Syrian Observatory for Human Rights said.

The Tabqa air base is the last position held by Syrian government troops in the northern Raqqa province, which is mostly under the control of the self-declared jihadist Islamic State group.

The Britain-based Observatory said the attack began overnight when a suicide bomber blew himself up at the entrance to the base, prompting clashes that killed 14 fighters.

The two sides exchanged rocket, artillery and heavy machinegun fire, and the army deployed air strikes against the attackers, the Observatory 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 fighters.

Those deaths brought Islamic State losses since it began its assault on Tabqa to at least 94, with more than 400 wounded, according to the Observatory.

The Islamic State group 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 the Islamic State seized the army's Brigade 93 and Division 17 posts in Raqqa, killing dozens of soldiers, some of whom it allegedly beheaded.

Meanwhile, the Syrian army airlifted reinforcements to the base and stepped up air strikes against Islamic State positions across Raqqa, using both precision rockets and barrel bombs.

A field commander told state news agency SANA that the Islamic State "were forced to retreat after they suffered heavy losses in personnel and equipment," and the army was in control over the adjacent town of al-Ejeil.

On Saturday afternoon, Syrian state television began broadcasting images from Tabqa showing soldiers it said were "ready to repel any new attack".

"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.