Islamic extremists reportedly killed at least 115 Syrian troops, guards and workers as they captured a gas field in central Syria following day-long clashes.

More than 20 militants also were said to have died in the fighting Thursday for the Shaer gas field, the deadliest battle so far between Islamic State fighters and forces loyal to President Bashar al-Assad.

In the past few weeks, Islamic State has seized a huge chunk of territory straddling the Iraq-Syria border and declared a caliphate. Earlier, they captured much of Syria’s oil-rich eastern province of Deir el-Zour.

The massive assault on the Shaer field, which lies in the desert region of Palmyra in the central province of Homs, began with a suicide bombing and an attack on army checkpoints Thursday, according to an activist there who goes by the name Beibares Tellawi.

The militants seized the field after fierce clashes with government forces that went on for 12 hours, Tellawi said Friday.

“This is the biggest operation [by Islamic State fighters] against the regime in Homs province,” Tellawi said, speaking via Skype from Homs.

The Britain-based Syrian Observatory for Human Rights said 115 people on the government side were killed.

The Observatory said 21 Islamic State fighters were killed in the battle, while Tellawi put the figure at 27.

An official in the Homs governor’s office contacted by the Associated Press said he had no information on the incident. The Syrian government rarely comments on deadly attacks targeting its forces.

The Observatory and Tellawi said the fate of more than 200 people captured in the facility was unknown.

An amateur video said to be from the area and released by activists shows dozens of bodies strewn across a desert area. It also shows jihadis with at least two multiple rocket launchers and two tanks, which they claim were sized in the fighting.

The video appeared genuine and corresponded to other AP reporting of the events.

A Web site commonly used by militants showed similar photos, including ones depicting green ammunition boxes allegedly captured by the “lions of the Islamic State” in the fighting around the gas field, as well as artillery and a multiple rocket launcher.

— Associated Press