Iraqi troops and militias drove Islamic State insurgents out of the town of al-Alam on Tuesday, clearing a final hurdle before a planned assault on Saddam Hussein's home city of Tikrit in their biggest offensive yet against the ultra-radical group.

The power base of executed former President Saddam's clan, Tikrit is the focus of a counteroffensive against Islamic State by more than 20,000 troops and Shiite Muslim militias known as Hashid Shaabi, backed by local Sunni Muslim tribes.

If Iraq's Shiite-led government is able to retake Tikrit it would be the first city clawed back from the Sunni insurgents and would give it momentum in the next, pivotal stage of the campaign — to recapture Mosul, the largest city in the north.