The offensive is the biggest military operation in the province since the Sunni Islamist radicals seized swaths of north Iraq last June and advanced towards the capital Baghdad, Rreuters reports.

Prime Minister Haider al-Abadi declared the start of the Salahuddin operations on Sunday during a visit to the government-held city of Samarra, where some of the thousands of troops and Shiite militia had gathered for the offensive.

The pace of their progress in Salahuddin could affect plans to recapture Mosul further north.

In Salahuddin, ISIS fighters control several strongholds including Tikrit, hometown of executed former Dictator Saddam and other Tigris river towns.

A source at the local military command said forces advanced north from Samarra towards the town of al-Dour, which officials describe as an ISIS bastion, and Tikrit, which lies about 40 km north of Samarra.

Iraq's air force was carrying out strikes in support of the advancing ground forces, who were being reinforced by troops and militia -- known as Hashid Shaabi, or Popular Mobilisation units -- from the neighbouring province of Diyala to the east.

Iraqi army forces in a military base just north of Tikrit also bombarded ISIS positions in the city, another source said.

Declaring the start of operations on Sunday evening, Abadi gave ISIS supporters what he said was one last chance to lay down their arms, or face "the punishment they deserve because they stood with terrorism".

But he also stressed that the army and militia must protect civilians and property in the battlefield.

In Iraq, months of air strikes, backed up by the Shi'ite militias, Kurdish peshmerga fighters and Iraqi soldiers have contained ISIS and pushed them back from around Baghdad, the Kurdish north, and the eastern province of Diyala.