The pilgrims were en route back to Iran from the Iraqi Shi'ite holy city of Karbala, where they had commemorated Arbaeen.
Baghdad: A suicide truck bomb killed about 73 people, most of them Iranian Shi'ite pilgrims, at a petrol station in the city of Hilla 100 km (62 miles) south of Baghdad on Thursday, police and medical sources said.
Islamic State (IS), the ultra hardline Sunni militant group that considers all Shi'ites to be apostates, claimed responsibility the attack in an online statement.
The group is also fighting off a U.S.-backed offensive on its stronghold Mosul, in northern Iraq, in which Iranian-trained Shi'ite militias are taking part.
The pilgrims were en route back to Iran from the Iraqi Shi'ite holy city of Karbala, where they had commemorated Arbaeen, the 40th day of mourning for the killing of Imam Hussein, a grandson of the Prophet Mohammad, in the 7th century AD, the medical sources said.
The gas station has a restaurant in its premises that is popular with travellers. Five pilgrim buses were torched by the force of the blast from the explosives-laden truck, a police official said.
Islamic State has intensified attacks over the past month in areas out of its control in efforts to weaken the offensive launched on Oct. 17 to retake Mosul, the last major city under IS control in Iraq.
Iran's Foreign Ministry condemned the attack without giving a casualties toll. Tehran will continue to support Iraq's ''relentless fight against terrorism," ministry spokesman Bahram Qasemi was quoted as saying by Tasnim news agency.