Iraqi officials have said a suicide car bomber struck a checkpoint in the town of Jurf al-Sakhar on October 27, a day after security forces pushed Islamic State (IS) militants out of the area.
Death tolls cited by various security and army sources ranged between 11 and 27.
Dozens of people were wounded.
Reuters quoted army and police sources as saying the victims were progovernment Shi'ite militiamen.
There were no immediate claims of responsibility for the attack.
On October 26, Iraqi soldiers backed by Shi'ite militia recaptured Jurf al-Sakhar, which had been in the hands of IS militants since late July.
The predominantly Sunni town is about 50 kilometers south of the capital, Baghdad.
Holding Jurf al-Sakhar is critical for Iraqi government forces who finally managed to drive out the Sunni extremist group after months of fighting.
Recapture of the town could allow Iraqi forces to prevent IS militants from edging closer to Baghdad.
It also severs IS militants' connections to their strongholds in western Anbar Province, and could stop them from infiltrating the mainly Shi'ite Muslim south.
Iraqi officials have said the insurgents have been moving fighters, weapons, and supplies from western Iraq though secret tunnels to Jurf al-Sakhar.
Jurf al-Sakhar lies on a road usually taken by Shi'ite pilgrims when they head in droves to the holy Shi'ite city of Karbala further to the south.
Pilgrims will be taking the route again next week in order to commemorate the death of Imam Hussein, one of the most revered Shi'ite figures.
Iraqi security forces also drove IS militants out of four villages near the oil city of Kirkuk in the north on October 26.
On the same day, Kurdish forces backed by U.S.-led air strikes regained control of the northern town of Zumar after weeks of fighting.
IS militants swept through the north of Iraq in June and control large parts of the west as well.
The extremist group also holds swathes of territory in neighboring Syria.
The militant group, made up of Arab and foreign fighters, seeks to create an Islamic caliphate.
U.S. President Barack Obama said earlier this month that some 60 countries have joined a U.S.-led coalition to counter Islamic State militants in what he predicted a "long-term campaign" against the extremist group.