Activists say Islamic extremists have launched an attack on a major air base in northeastern Syria, aiming to seize the last position held by the Syrian government in a province that is an Islamic State stronghold. 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 has virtually eliminated the military’s presence in the Raqqa province, with the exception of Tabqa. Wednesday’s attack was announced on militant websites. It was also reported by the Raqqa Media Center, an activist collective, which reported fierce clashes around the facility accompanied by government airstrikes. The Britain-based Syrian Observatory for Human Rights also reported the clashes.