If confirmed, the recovery of the Baiji facility could provide critical momentum for government forces charged with restoring stability in a country facing its worst security crisis since dictator Saddam Hussein was toppled in 2003.
"The first Iraqi force, the anti-terrorism force called Mosul Battalion, entered Baiji refinery for the first time in five months," police colonel Saleh Jaber from the Baiji refinery protection force told Reuters.
State television flashed news of the advance and broadcast footage it said was of Iraqi security forces entering the refinery's gate.
"In this area, terrorists were stationed to the left and right. If God is willing, Baiji will be the main key to liberating each span of Iraq," the commander of provincial security operations, Abdel Wahab al-Sa'adi, told the broadcaster.
US-led air strikes have prevented the militant group, which swept through northern Iraq in June almost unopposed by the Iraqi army, from making significant further territorial gains for its self-proclaimed caliphate.
ISIL seized the city of Baiji and surrounded the sprawling refinery during that first advance in June.