Islamic extremists fired rockets and tank shells 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 group, activists said.

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 recent months has virtually eliminated the presence of President Bashar Assad’s military 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. […]