Beirut: Islamist militants fired rockets and tank shells on Wednesday at a major air base in northeastern Syria, kicking off a long-anticipated offensive to seize the last position held by the Syrian government in a province that is a stronghold of the Islamic State of Iraq and the Levant (Isil), activists said.

The attack on the Tabqa air base had been expected for weeks. Isil militants 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.

Last month, the militants overran the sprawling Division 17 military base in Raqqa, killing at least 85 soldiers. Two weeks later, Isil seized the nearby Brigade 93 base after days of heavy fighting.

Militant websites affiliated with Isil announced the assault on Wednesday. Since July, following their blitz in Iraq and after they declared a self-styled caliphate straddling the Iraq-Syria border, Isil militants 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 Centre, an activist collective, which reported fierce clashes around the facility accompanied by government airstrikes.

It said army warplanes conducted airstrikes on suspected militant positions in the nearby town of Tabqa on the Euphrates river, which flows from Turkey through Syria into Iraq.

The town is home to Al Furat dam, Syria’s largest, now controlled by Isil.

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.