Iraqi troops launch anti-Isis operation in Tikrit
Iraqi security forces and Shi'ite fighters gesture from the top of an armoured vehicle as they gather at Udhaim dam, north of BaghdadReuters

The Iraqi army has started a long-awaited military offensive against Islamic State (Isis) militants, attacking the key city of Tikrit, Saddam Hussein's hometown.

State TV said government forces supported by allied Shi'ite and Sunni militias initiated operations to retake the city some 130km (80 miles) north of Baghdad, on the road to Iraq's second largest city, Mosul.

IS positions were pounded by air strikes and artillery fire ahead of the attack, which came from different directions, Al-Iraqiya television reported.

Tikrit was seized by IS last summer, as the extremist group conquered large swathes of the country in a lightning campaign backed by Baathists and other former loyalists to Hussein.

Hours before the operation was launched, Prime Minister Haider al-Abadi urged local Sunni tribes and Saddam loyalists to desert IS, saying it was their "last chance" to do so and receive pardon.

"I call upon those who have been misled or committed a mistake to lay down arms and join their people and security forces in order to liberate their cities," al-Abadi said, speaking in the nearby city of Samarra where he arrived on Sunday (1 March) to supervise the attack.

"This is their last chance. If they insist on staying on their wrong path they will receive the fair punishment they deserve because they ... stood with terrorism," he said, according to Reuters.

Retaking Tikrit is of high strategic and symbolic importance to Baghdad.

In February, US officials disclosed that a coordinated military mission to retake Mosul involving some 25,000 Iraqi troops and Kurdish fighters was being set up and expected to start in April or May.

The seizing of Tikrit is a necessary prerequisite of such an operation as the city is located on the main northbound artery accessing the Mosul region.

The city was also the set of the massacre of hundreds of Iraqi soldiers who were brutally executed on camera by IS after being captured from the nearby Camp Speicher last summer.

Avenging the mass execution would likely add to government troops' motivation in the current assault, which could however prove difficult.

"Isis has entrenched itself in Tikrit and will most likely mount strong resistance," the Washington-based Institute for the Study of War (ISW) said.

"In addition, Isis has likely booby-trapped the city with IEDs [improvised explosive devices], further complicating the movement of the anti-Isis forces."