Iraqi government forces have launched a massive operation to reclaim a major city from the Islamic State.

According to Yahoo! News, a force of more than 30,000 Iraqi government forces and militia fighters from the Popular Mobilisation units (PMUs) launched operations on Monday to liberate Tikrit and surrounding areas in the Salahuddin province from the control of the Islamic State. 

PMUs are composed of Iraqi factions that previously resisted the US-led invasion that ended the rule of Saddam Hussein in 2003.

Tikrit on the Tigris River is Hussein's hometown. The Islamic State attacked the city with help from the Army of the Men of the Naqshbandi Order in June last year in what became known as the "Northern Iraq Offensive" and eventually gained control of the city on June 11.

The Army of the Men of the Naqshbandi Order is a militant Sufi Muslim organisation, and is one of many groups that engaged in the insurgency against the new Iraqi government following the defeat of Hussein.

Government forces subsequently tried to retake the city from the militants after its fall, but had to withdraw in July. 

According to a security official, who spoke on condition of anonymity, the attacking force is made up of military personnel, police, counter-terrorism units and local Sunni tribes that are hostile to the Islamic State, in addition to the PMUs.

The official also revealed that sizeable air support as well as artillery are being called in to assist the offensive.