Car bomb attacks killed at least 28 government-backed militiamen and wounded 36 in Iraq on Tuesday, a security source said.The attacks took place near the Islamic State-held town of Baiji in Iraq's province of Salahudin.The four suicide attacks occurred as the Iraqi security forces and allied militias, known as Hashd Shaabi, or Popular Mobilisation, were advancing to recapture the IS-seized town, Xinhua quoted the source as saying.The powerful blasts stopped the advance of the troops and allied militias, while heavy clashes continued, the source said.The battles in Baiji came two days after the town's mayor, Mohammed Hammed, said the troops freed several villages outside the town and were preparing to launch an offensive to recapture it.Since March 2, some 30,000 Iraqi troops and thousands of allied Shia and Sunni militias are involved in Iraq's biggest offensive to recapture the northern part of Salahudin province, including Tikrit and other key towns and villages from IS militants.