Military commanders said the army and mostly Shiite militia forces had retaken the town of al-Dour on Tikrit's outskirts, known outside Iraq as the area where executed former dictator Saddam was found hiding in a pit near a farm house in 2003.
Hadi al-Amiri, leader of the largest Shiite militia group taking part in the operation, said al-Dour had been "totally liberated" and that the advance on another key town north of Tikrit, al-Alam, would take place on Saturday.
The army, joined by thousands of Shiite militiamen, is five days into an advance on Saddam's home city of Tikrit, by far the biggest target yet in a campaign to roll back last year's advance by ISIS fighters.
Iraqi Golden Forces
The assault by the Shiite-led army and its militia allies on Tikrit in Iraq's Sunni heartland has symbolic importance for both sides.
Officials said on Friday they had captured a farm to the east of Tikrit that belonged to Saddam's deputy Ezzat Ibrahim al-Douri, now a prominent ally of the terrorists fighters.
"Hadi al-Amiri, leader of the largest Shiite militia group taking part in the operation, said al-Dour had been "totally liberated" and that the advance on another key town north of Tikrit, al-Alam, would take place on Saturday."
"There are areas that we have liberated so we must place forces in these areas to prevent ISIS insurgents from returning," said a source at the Samarra operations command to the south of Tikrit. "It is a reorganization not a standstill."
The outcome of the Tikrit offensive will have a significant bearing on the shape and timing of an even more pivotal move further north against Mosul, the largest city under ISIS control.
Rapid military success would give government in Baghdad momentum as well as foothold in the north, while stalemate in Tikrit would stall any future plans.
An official from the Hashid Shaabi, or Popular Mobilisation units as the mainly Shiite militias are known, said they had provided food and water and safe passage to civilians in territory taken this week.
"Most of them were sheep and cow herders, nomads, the official told Reuters.
To the south of Tikrit, ISIS militants attacked soldiers and militia fighters east of the city of Samarra with an explosives-laden tanker, killing four people and wounding 28, a source at Samarra hospital said.