The attack on the Tabqa air base had been expected for weeks. Islamic State fighters have tightened their siege of the sprawling facility in recent days, capturing a string of nearby villages.
The group in past months virtually eliminated the military's presence in Raqqa province, with the exception of Tabqa. The air base is one of the most significant government military facilities in the area, containing several warplane squadrons, helicopters, tanks, artillery and ammunition.
Militant websites affiliated with the Islamic State announced the assault today. Since July, following their blitz in Iraq and after they declared a self-styled caliphate straddling the Iraq-Syria border, Islamic State fighters methodically have gone after isolated government bases in northern and eastern Syria, killing and decapitating army commanders and pro-government militiamen.
The Tabqa attack also was reported by the Britain-based Syrian Observatory for Human Rights and the Raqqa Media Center, an activist collective, which reported fierce clashes around the facility accompanied by government airstrikes.
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The group's lightning advance has brought under its control territory stretching from northern Syria as far as the outskirts of Baghdad in central Iraq.
The militant gains brought US forces back into conflict in Iraq for the first time since they withdrew in 2011.
Washington began carrying out dozens of airstrikes against militant targets in Iraq on August 8.
The beheading marks the first time the Islamic State has killed an American citizen since the Syrian conflict broke out in March 2011, upping the stakes in an increasingly chaotic and multilayered war.