Sources told Al Jazeera on Wednesday that gunment had set up checkpoints around the city, which is the capital of Salaheddin province and lies about halfway between Mosul and the capital Baghdad.
“All of Tikrit is in the hands of the militants,” a police colonel told the AFP news agency.
A police brigadier general told AFP that fighters attacked from the north, west and south of the city, and that they were from ISIL.
A police major told the agency that the militants had freed about 300 inmates from a prison in the city.
The city’s capture follows the fall of Mosul on Tuesday. Half a million people are believed to have fled Mosul, a quarter of the city’s population, since the takeover.
The Geneva-based International Organisation for Migration said the takeover had “displaced over 500,000 people in and around the city”.
Al Jazeera’s Imran Khan, reporting from Baghdad on Wednesday, said aid agencies were under pressure to deliver humanitarian aid as nobody had expected Mosul to fall quite so dramatically.
Earlier on Wednesday, ISIL advanced into the oil-refinery town of Baiji before Iraq’s Fourth Armoured Divison forced the group to retreat.
The group had threatened local police and soldiers not to challenge them and warned the town’s most prominent tribal leaders to lay down their weapons.
“We are coming to die or control Baiji, so we advise you to ask your sons in the police and army to lay down their weapons.”