Irbil , Iraq ( CNN ) ISIS claimed it controlled part of Iraq 's largest oil refinery Sunday , posting images online that purported to show the storming of the facility , fierce clashes and plumes of smoke rising above the contested site . The group said it 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 Iraqi forces remain in full control . CNN could n't independently verify ISIS ' claim . It would n't be the first time that militants and Iraqi forces have battled over the refinery , a key strategic resource that has long been a lucrative target because the facility refines much of the fuel used by Iraqis domestically . If an attack damaged oil fields or machinery , it could have a significant impact . The refinery is just 40 kilometers ( 25 miles ) from the northern Iraqi city of Tikrit , which Iraqi forces and Shiite militias wrested from ISIS less than two weeks ago . CNN 's Jennifer Deaton and Catherine E. Shoichet contributed to this report .
ISIS says it controls several buildings at the Baiji oil refinery . Iraqi government security officials say Iraqi forces remain in full control . The refinery , Iraq 's largest , has long been a lucrative target for militants .