At least 47 people have died after a fuel tanker was blown up at a checkpoint near Hilla, a city south of the Iraqi capital Baghdad.
Thirty-nine of the dead were civilians, the rest police, one report said. Dozens were injured in the blast.
The attack happened shortly after noon local time (0900 GMT) while dozens of cars were at the checkpoint.
The so-called Islamic State said it had carried out the attack, in a post on the pro-IS Amaq news agency website.
"A martyr's operation with a truck bomb hit the Babylon Ruins checkpoint at the entrance of the city of Hilla, killing and wounding dozens," Reuters quoted the group's statement as saying.
Though the area where the attack took place - south of Baghdad - is not controlled by IS, the Sunni extremists have been targeting areas away from the fighting.
Suicide truck bombs, loaded with explosives, are increasingly being used by IS to hit softer targets and to deliberately cause civilian casualties - particularly in Shia-dominated areas.
United Nations figures show that at least 670 Iraqis were killed last month, two thirds of them civilians.
BBC defence correspondent Jonathan Beale, in Baghdad, says the fear is that IS will step up such attacks as they lose ground in the rest of the country.
Iraqi security forces backed by the US-led coalition are stepping up the fight in and around the towns and cities still held by IS.
They are now preparing for a major offensive on Mosul, IS's headquarters in Iraq.