Islamic State group fighters seized a gas field in eastern Syria Thursday, marking a significant victory for the militants, who have seen their territory shrink after months of air bombings led by Russia and the United States. The capture of the main Shaer gas field in the Palmyra desert comes after the militants, also known as ISIS, lost the ancient city in March to the Syrian regime.

The ISIS fighters claimed they took the gas field area after killing at least 30 Syrian troops and seizing large caches of heavy weapons, including tanks and missiles, during a three-day assault, Reuters reported. The Syrian Observatory for Human Rights confirmed the seizure of the gas field area roughly 90 miles northwest of Palmyra, the ancient city known for its historic monuments.

In December, ISIS ruled over nearly two-thirds of Syria’s oil production capacity. The group used the revenue made from selling oil to small, privately owned refineries to fund its conquests and pay its fighters, to the tune of about $2 million to $3 million a day in total.

Foreign forces have forced ISIS to retreat across Syria and Iraq in recent months. In the militant’s stronghold in eastern Syria, the Syrian and Russian governments recaptured Palmyra in March after a 10-month battle that saw the destruction of the iconic temples of Bel and Baalshamin as well as the Arch of Triumph.

“We have restored security and stability to the city of Palmyra, and established full control on the surrounding, commanding hills,” the Syrian military said in a statement at the time.

The U.S. began bombing ISIS territory in Iraq in August 2014 and in Syria in September 2014. The group has lost about 40 percent of the territory it once held in Iraq and about 10 percent of the populated territory it once controlled in Syria, the White House has said.