Syrian television showed footage of soldiers running and deploying in a vast desert area which it said was the Sha'ar gas field. The army said in a statement it retook the field after a "precise operation in which dozens of terrorists were killed."
However, a source from the Islamic State said the fighters pulled out after destroying the field's equipment and capturing at least 15 tanks and dozens of rockets which were used to guard the field.
"We pulled out because it was no longer good for us to stay. The goal was to get the tanks and rockets present at the field and we did," he said. "There is no point in staying there and become an easy target for the regime and its warplanes."
Islamic State, previously known as the Islamic State in Iraq and Levant, has advanced in Syria and taken over swaths of territory in neighboring Iraq in what it has described as a bid to establish an Islamic caliphate.
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During the assault on the field, the Islamic State fighters killed at least 270 soldiers, guards and staff and also killed at least 85 soldiers when they captured an army base in the province of Raqaa on Friday, the Observatory said. The fighters said they decapitated most of the soldiers and hung several heads outside the base gates.