Many of the dead in yesterday's attack near the city of Nasiriyah were Shiite Muslim pilgrims, some of them Iranian, officials said.
"The death toll has risen to 84 after the discovery of 10 more bodies at the scene of the attack," said Jassem al- Khalidi, health director for Dhiqar province, which has largely been spared the violence that has plagued northern and central Iraq.
"Another 93 people were wounded, many of them seriously," Khalidi told AFP.
They left a trail of destruction, with charred bodies scattered on the ground near the burnt-out wrecks of cars, buses and trucks, an AFP correspondent reported.
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The attack was quickly claimed by IS, which appears to be switching to insurgent attacks after suffering a string of setbacks on the battlefield.
UN envoy Jan Kubis condemned the "cowardly twin attacks... Which resulted in numerous civilian casualties, including many pilgrims."
Shiites have been the target of repeated attack by the Sunni extremists of IS who regard them as heretics.