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 attack killed at least 25 people and wounded more than 100 others, according to a statement from Iraq's Oil Ministry.