Baghdad, Iraq (CNN)Hundreds of additional Iraqi troops are being sent to reinforce colleagues who are trying to fend off ISIS' attempt to overrun Iraq's largest oil refinery, a key paramilitary force said Tuesday. The reinforcements come four days after ISIS began attacking northern Iraq's Baiji oil refinery, a key strategic resource that has long been a target because the facility refines much of the fuel used by Iraqis domestically. The additional troops came from Camp Speicher, a fortified Iraqi base near the city of Tikrit, according to the media office of the Hasd Al-Shaabi militia. The reinforcements include two federal police regiments, an Iraqi military quick reaction force battalion and a regiment from Hasd Al-Shaabi, which is a predominantly Shia militia that worked with the Iraqi military as well as Sunni fighters to liberate Tikrit from ISIS about two weeks ago. ISIS launched an assault on the Baiji oil refinery late Saturday. By Sunday, ISIS said its fighters were inside the refinery and controlled several buildings, but Iraqi government security officials denied that claim and insisted that Iraqi forces remained in full control. The Hasd Al-Shaabi media office said Tuesday that Iraqi troops already at the refinery were holding their ground, preparing to push ISIS out of the facility entirely. The attack could have a significant effect if it damages oil fields or machinery. The refinery is 40 kilometers (25 miles) from Tikrit. CNN's Hamdi Alkhshali reported from Baghdad. CNN's Jason Hanna wrote in Atlanta. CNN's Arwa Damon and Catherine E. Shoichet contributed to this report.
ISIS attacked the Baiji oil refinery Saturday .
The refinery, Iraq's largest, has long been a lucrative target for militants .