ISIS targets Iraq's largest oil refinery.

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 couldn't independently verify ISIS' claim. It wouldn't be the first time that militants and Iraqi forces have battled over the refinery, a key strategic resourcethat 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.

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