More than a dozen Iraqi workers at a small crude-oil station near the northern city of Kirkuk were reported missing Saturday, after Islamic State group militants overran the facility, Reuters reported. Although Kurdish military forces managed to push back the jihadist fighters in Kirkuk, the attack signals a new effort to capture the strategic city, which is on the way to Erbil, an Iraqi economic powerhouse where many U.S. and European diplomats reside.

“We received a call from one of the workers saying dozens of [Islamic State] fighters were surrounding the facility and asking workers to leave the premises. We lost contact and now the workers might be taken hostage,” an engineer with the state-run North Oil Co. told Reuters Saturday, confirming 15 workers at the crude-oil separation unit in Khabbaz were missing.

Forces of the Islamic State group, formerly known as either ISIL or ISIS, seized parts of the Khabbaz oil fields after attacking the Kurdish militia known as the peshmerga in Kirkuk Friday. Khabbaz, about 12 miles southwest of Kirkuk, produced a maximum capacity of 15,000 barrels per day, Reuters said.

The jihadist insurgents seized swaths of oil-rich areas in northern Iraq last summer, and they have been selling crude oil and gasoline to finance their robust operations.

David S. Cohen, the U.S. Treasury Department’s undersecretary for terrorism and financial intelligence, said in October that the Islamic State group is “the best-funded terrorist organization” America has “ever confronted,” as indicated in a video hosted by Carnegie Endowment for International Peace.

Kurdish military officials reported at least 25 of their solders were killed in the attack by the Islamic State group Friday. The casualties were the results of one of the most aggressive attacks on the Iraqi Kurds in months, but peshmerga forces in the oil-rich region surrounding Kirkuk fought back Saturday. “ISIL fighters took advantage of the fog, and they launched their surprise attack, but we managed to defeat them,” Mariwan Abdel Khaleg, a member of the Peshmerga 17th Brigade, told Al Jazeera Saturday. “We are responsible for protecting Iraqi territory, inch by inch.”

The Islamic State group insurgency now controls about one-third of Iraq and almost one-third of Syria, according to the Associated Press. The Sunni militant group seeks to establish an Islamic caliphate over all Muslims in Iraq and Syria.