Iraqi forces and mainly Shi'ite militia battling to wrest full control of the city of Tikrit from Islamic State militants paused their offensive for a second day on Saturday as they awaited reinforcements.

A source in the local military command center told Reuters military commanders had "reached a decision to halt the operation until a suitable, carefully set plan is in place" to break into central Tikrit.

The source, speaking by phone from near Tikrit, said the Iraqi forces and Iranian-backed Shi'ite militias known as Hashid Shaabi (Popular Mobilisation) were waiting for reinforcements from "well-trained forces". He did not give a timeline for the arrival of the reinforcements.

"We do not need a large number, just one or two thousand. We need professional personnel and soldiers," he said, explaining they were needed to engage in street-by-street battles with Islamic State fighters who have booby trapped many buildings in the city and laid improvised explosive devices and roadside bombs.

Army and militia forces pushed into Saddam Hussein's home city this week in their biggest drive yet against the insurgents who seized large swathes of land in Iraq and neighboring Syria last year in a lightning campaign halted just outside Baghdad.

More than 20,000 troops and allied militias entered the city about 160 km north of the capital on Wednesday after retaking towns to the south and north in a campaign launched nearly two weeks ago.-Reuters