U.S. airstrikes against the Islamic State (IS) have intensified in recent weeks, as fighter jets and drones conduct targeted bombings of IS-controlled oil refineries in Syria.

U.S. Central Command released a report which said U.S. military forces, Saudi Arabia and the United Arab Emirates jointly “conducted 13 airstrikes against 12 ISIL (Islamic State of Iraq and the Levant) controlled modular oil refineries” on Sept. 24. The coalition targeted oil refineries and one ISIL vehicle located in eastern Syria, according to the report.

The U.S. has begun to target oil refineries under control of the IS, also known as ISIL, in an attempt to limit the group’s resources. These refineries have been financing their terrorist organization, generating $2 million a day in revenue, according to the Pentagon.

According to BBC, IS controls six of the 10 oil fields in Syria and produces roughly 50,000 barrels in Syria and 30,000 barrels in Iraq a day.

The oil produced from these fields is then sold to local merchants who smuggle it across the borders to Turkey, Iran and Jordan or it is sold back to the Syrian government, reported the BBC.

The recent airstrikes were a part of President Obama’s strategy to dismantle the IS, or as Obama calls it, the “network of death.” The report stated the strikes were believed to be a success, destroying “small-scale refineries” that produced 300-500 barrels of refined petroleum a day.

The recent U.S. military bombing campaign in Syria to take out IS and the Khorasan group, a terrorist group from the region in league with al-Qaeda, is part of a larger campaign. At a press conference with Pentagon officials, Lt. Gen. William Mayville said that this campaign will mostly likely take place for the next couple of years.