A spokesman for the Ministry of Defence, A'laa al-Idani, told IBTimes UK that air strikes by the US and its coalition allies would help to destroy the network of tunnels and allow the army and militias to fully take the city after over a week of vicious fighting.

The ISIS group seized the Sunni city last summer during its lightning advance across northern and western Iraq.

The battle for Tikrit is seen as a key step toward eventually driving the ISIS group from Iraq's second largest city Mosul, which is further north.

The US first launched airstrikes to reinforce Iraq's embattled military in August.

A spokesman for Iraq's Popular Mobilisation Units, the vast majority of which consists of Shiite militias, told AP on Thursday that a number of Shiite militias, including Asaib Ahl al-Haq, Ketaeb Hizbollah and Muqtada al-Sadr's Peace Brigades are boycotting the Tikrit operation because of the involvement of US forces.

But Iraqi PM has another view point."The Popular Mobilisation Units are part of the security forces. They are linked to the Commander in Chief of the Armed Forces. As I am responsible for the army and the police forces as commander in chief, I'm also responsible for the Popular Mobilisation Units," Iraqi Prime Minister Haider al Abadi said in