Syrian troops battled Wednesday to hold onto their last stronghold in the northern province of Raqa, under fierce attack from Islamic State jihadists, a monitoring group said.
The Syrian Observatory for Human Rights said IS fighters had been attacking Tabqa military airport for around 10 days, but the fighting overnight from Tuesday was the heaviest yet.
At least six jihadists have been killed in the fighting, which has involved medium and heavy weaponry as well as government air raids.
The air force carried out 15 raids Wednesday around Tabqa, sending hundreds of residents into flight.
Tabqa airport is the last remaining stronghold of Syrian troops in Raqa province, which has been largely overrun by IS fighters.
The extremist group controls the provincial capital, which was captured by other rebel groups in March 2013 but later seized by IS.
In early August, it captured the key Brigade 93 army base in Raqa, not long after taking the military's Division 17, where at least 85 people were killed in fighting or summarily executed.
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Elsewhere on Wednesday, Kurdish fighters clashed with IS militants in Hasakeh province in northeastern Syria, the Observatory said.
It said IS seized control Tuesday of Jazza village near the Iraqi border in fighting that killed 10 Kurdish combatants and four jihadists.
The latest IS advances in Raqa and Aleppo province, also in northern Syria, have prompted the Syrian government to launch intensive air strikes against IS for the first time.
Rebel groups have often accused the regime of avoiding attacks on IS because the Sunni extremist group has battled more moderate rebel forces.
The IS emerged from the Iraqi affiliate of Al-Qaeda but has since rejected its leadership.
It initially worked alongside other rebel groups in Syria, but they turned against it because of its abuses and its bid to dominate seized territory.
But it continues to draw recruits, and on Twitter its supporters expressed delight about the attack on the Tabqa base.
"The lions of the Islamic State have announced a war without mercy. It is time to cut of the heads of the Nussayris," one supporter tweeted, using a pejorative term for Alawites, an offshoot of Shiite Islam to which President Bashar al-Assad belongs.