Hilla - A truck bomb exploded at a crowded checkpoint outside the city of Hilla, south of Baghdad, on Sunday, killing at least 47 people, officials and medical sources said.Faleh al-Radhi, the head of the security committee at Babil provincial council, said "the attack was carried against a checkpoint at the northern entrance to Hilla."A doctor at Hilla hospital put the number of people killed by the blast at 47, including around 20 members of the security forces, and said at least 72 people were also wounded.Radhi and police officers confirmed the casualty toll, the heaviest from any car bomb attack in Iraq this year.Officials said the vehicle was a truck packed with explosives and was detonated after being pulled over by checkpoint security as it tried to enter Hilla.Pictures posted on social media showed vast destruction around the checkpoint, where cars are usually bumper-to-bumper at that time of day, queueing to be checked by security personnel.A doctor at Hilla hospital said at least 11 of the wounded were in a very serious condition.The Islamic State jihadist group claimed responsibility for the attack. In a statement posted on social media, ISIS named the bomber who detonated his explosives-laden truck at a checkpoint outside the city of Hilla as Abu Islam al-Ansari."The Rafidha [a derogatory term for Shi'ites]) must understand that the battle has just begun and that the worst is yet to come," the statement said.The group claimed that the explosion had killed and wounded "more than 90" people. The attack was the deadliest bombing by ISIS in Iraq this year.A March 2014 suicide bombing at a checkpoint on the outskirts of Hilla, 80km south of the capital, killed 50 people and wounded more than 150.When Iraqi forces began their counter-offensive against ISIS in late 2014, securing the Shi'ite shrine cities of Najaf and Karbala, south of Baghdad, was a priority.The jihadist group has been losing territory in Iraq for almost a year. In the most recent operation, Iraqi forces are retaking areas west of the city of Samarra.In the cities the group retains control over, internal tension appears to be on the rise and the lack of supplies is taking its toll.Observers have warned that, as their self-proclaimed "caliphate" shrinks towards extinction, ISIS fighters are likely to revert to their old guerrilla tactics and ramp up suicide car bomb attacks on civilian targets.