The Reuters news agency reported on Wednesday that a car rigged with explosives drove into the checkpoint in al-Qaim district as it was jointly manned by the Iraqi army and Shia militias.
The town’s mayor, Ahmed al-Mehlawy, said five militiamen and three civilians were killed in the attack.
However a security source told the dpa news agency that the death toll was higher, saying 21 people were killed, 15 of those militiamen.
No group immediately claimed responsibility for the attack but fighters from the Islamic State of Iraq and the levant (ISIL, also known as ISIS) operate in the area.
Al-Qaim, a border town of around 150,000 people, has previously served as a crossing point for fighters, weapons and money used to fuel the armed group.
ISIL once controlled significant portions of Iraq and Syria, ruling over roughly 110,000sq km in those two countries.
But since December, after Prime Minister Hyder al-Abadi declared victory over the armed group, the group’s territory is said to about 10,000sq km, mostly in eastern Syria.
According to a recent United Nations report, there are between 20,000 and 30,000 ISIL members in Iraq and Syria, despite the groups’ defeat and a halt in the flow of foreigners joining its ranks.