Islamic State (ISIS) terrorists on Thursday captured a major gas field in eastern Syria, marking the first territorial expansion for the jihadists in the Palmyra desert area since they lost the ancient city in March.

ISIS seized the main Shaer gas field, according to rebel sources and the Syrian Observatory for Human Rights as cited by Reuters.

The Amaq news agency affiliated with ISIS said they took over the gas field from Syrian regime troops, killing at least 30 of them and seizing large stockpiles of heavy weapons such as tanks and missiles in the process.

That seizure was confirmed by the Syrian Observatory monitor group, which noted the field lies around 150 kilometers (over 90 miles) northwest of Palmyra, and added that ISIS conquered the field after three days of fighting and despite regime air support targeting them.

ISIS gained prominence in early 2014 as it seized numerous oil and gas fields and banks, becoming the wealthiest terrorist organization in the world.

Reuters reports that the Shaer gas field and nearby gas facilities in Homs province previously were conquered by ISIS several times, but they last lost hold of the site in late 2014. Since then the jihadists have continued attacking Syrian regime forces in the area.

ISIS is said to be earning $2 million a day in black market oil sales, and the seized resources have been key in propping up the terrorist organization.