Islamic State (Isis) militants are grimly holding on to an area near the city centre of Tikrit as Iraqi forces boosted by Shiite militia close in.

About three quarters of Tikrit have come under the control of the security forces. About 150 IS fighters are trying to hold on to parts of the strategic town, Main al-Kadhimi, chief of the key paramilitary force, Hashd al-Shaabi, leading the offensive against IS, told CNN.

Iraqi defence minister Khaled al-Obeidi has said the next phase of the anti-IS operation would begin one Tikrit has been secured. Taking Tikrit will be a boost to the Iraqi forces' morale.

This would place the anti-IS campaign in a strong position to push for the main IS stronghold of Mosul, which is ten times bigger than Tikrit.

"We don't want to be rushed because we want to avoid casualties. Tikrit is sealed off from all sides," Police Staff Major General Bahaa al-Azzawi told AFP from Albu Ajil, a small village near Tikrit.

Reports suggest the Iraqi forces and Shiite fighters are waiting for the IS extremists to surrender in order to avoid further casualties.

Ever since the town fell to the IS in June, Iraqi forces had made several attempts to retake it but failed. The latest offensive is the largest operation targeting Tikrit in which nearly 30,000 anti-IS forces are involved.